Slides from my BSDCan Talk

These are the slides from by BSDCan talk on a system-level public-key trust system for FreeBSD.

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Democratizing and Decentralizing Social Media Infrastructure

This post is more of a release announcement for a whitepaper I’ve been working on for some time now.  After a number of edits, I am prepared to release the full paper for public reading.  I will be seeking to present this in the form of a talk at open-source, particularly socially-focused conferences.

The central thesis lies around an alternative infrastructure for (among other things) social applications that is fundamentally decentralized, democratic, and user-respecting.  No, this is not a blockchain proposal!  While I claim no credit for it, the timing really couldn’t have been better, with the news about FaceBook, Cambridge Analytica, and others breaking daily.

In the coming year, I will be seeking to implement the architecture I describe herein.  I’ll be publicizing the paper, seeking non-profit funding and industry partners, and developing an open-source reference implementation.

But rather than keep talking, I’ll leave you to read the paper.  Comments and other points of interest are welcome, and longer-form comments should be directed to me via email.

A Potential Meltdown/Spectre Countermeasure

I posted a proposed countermeasure for the meltdown and spectre attacks to the freebsd-security mailing list last night.  Having slept on it, I believe the reasoning is sound, but I still want to get input on it.

MAJOR DISCLAIMER: This is an idea I only just came up with last night, and it still needs to be analyzed.  There may very well be flaws in my reasoning here.

Countermeasure: Non-Cacheable Sensitive Assets

The crux of the countermeasure is to move sensitive assets (ie. keys, passwords, crypto state, etc) into a separate memory region, and mark this non-cacheable using MTRRs or equivalent functionality on a different architecture.  I’ll assume for now that the rationale for why this should work will hold.

This approach has two significant downsides:

  1. It requires modification of applications, and it’s susceptible to information leaks from careless programming, missing sensitive assets, old code, and other such problems.
  2. It drastically increases the cost to access sensitive assets (a main-memory access), which is especially punitive if you end up using sensitive asset storage as a scratchpad space

The upside of the approach is that it’s compatible with a move toward storing sensitive assets in secure memory or in special devices, such as a TPM, or the flash device I suggested in a previous post.

Programmatically, I could see this looking like a kind of “malloc with attributes” interface, one of the attributes being something like “sensitive”.  I’ll save the API design for later, though.

Rationale

In this rationale, I’ll borrow the terminology “transient operations” to refer to any instruction or portion thereof which is being executed, but whose effects will eventually be cancelled due to a fault.  In architecture terminology, this is called “squashing” the operation.  The rationale for why this will work hinges on three assumptions about how any processor pipeline and potential side-channel necessarily must work:

  1. Execution must obey dependency relationships (I can’t magically acquire data for a dependent computation, unless it’s cached somewhere)
  2. Data which never reaches the CPU core as input to a transient operation cannot make it into any side-channel
  3. CPU architects will squash all transient operations when a fault or mispredicted branch is discovered as quickly as possible, so as to recover execution units for use on other speculative branches

Defeating Meltdown

The meltdown attack depends on being able to execute transient operations that depend on data loaded from a protected address in order to inject information into a side-channel before a fault is detected.  The cache and TLB states are critical to this process.For this analysis, assume the cache is virtually-indexed (see below for the physically-indexed cache case).  Break down the outcomes based on whether the given location is in cache and TLB:

  • Cache Hit, TLB Hit: You have a race between the TLB and cache coming back.  TLBs are typically smaller, so they are unlikely to come back after the cache access.  This will detect the fault almost immediately
  • Cache Hit, TLB Miss: You have a race between a page-table walk (potentially thousands of cycles) and a cache hit.  This means you get the data back, and have a long time to execute transient operations.  This is the main case for meltdown.
  • Cache Miss, TLB Hit: The cache fill operation strongly depends on address translation, which signals a fault almost immediately.
  • Cache Miss, TLB Miss: The cache fill operation strongly depends on address translation, which signals a fault after a page-table walk.  You’re stalled for potentially thousands of cycles, but you cannot fetch the data from memory until the address translation completes.

Note that both the cache-miss cases defeat the attack.  Thus, storing sensitive assets in non-cacheable memory should prevent the attack.  Now, if your cache is physically-indexed, then every lookup depends on an address translation, and therefore, fault detection, so you’re still safe.

A New Attack?

In my original posting to the FreeBSD lists, I evidently had misunderstood the Spectre attack, and ended up making up a whole new attack (!!)  This attack is still defeated by the non-cacheable memory trick.  The attack works as follows:

  1. Locate code in another address space, which can potentially be used to gather information about sensitive information
  2. Pre-load registers to force the code to access sensitive information
  3. Jump to the code, causing your branch predictors to soak up data about the sensitive information
  4. When the fault kicks you back out, harvest information from branch predictors

This is defeated by the non-cacheable store as well, by the same reasoning.

Aside: this really is a whole new class of attack.

High-Probability Defense Against the Spectre Attack

The actual spectre attack relies on causing speculative execution of other processes to cause cache effects which are detectable within our process.  The non-cacheable store is not an absolute defense against this, however, it does defeat the attack with very high probability.

The reasoning here is that any branch of speculative execution is highly unlikely to last longer than a full main memory access (which is possibly thousands of cycles).  Branch mispredictions in particular will likely last a few dozen cycles at most, and execution of the mispredicted branch will almost certainly be squashed before the data actually arrives.  Thus, it can’t make it into any side-channel.

Summary

This is a potential defense against speculative execution-based side-channel attacks, which is based on restoring the dependency between fault detection and memory access to sensitive assets, and incurring a general access delay to sensitive assets.

This blocks any speculative branch which will eventually cause a fault from accessing sensitive information, since doing so necessarily depends on fault detection.  This has the effect of defeating attacks that rely on speculative execution of transient operations on this data before the fault is detected.

This also defeats attacks which observe side-channels manipulated by speculative branches in other processes that can be made to access sensitive data, as the delay makes it extremely unlikely that data will arrive before the branch is squashed.

Future

Assuming the reasoning here is sound I plan to start working on implementing this for FreeBSD immediately.  Additionally, this defense is a rather coarse mechanism which repurposes MTRRs to effectively mark data as “not to be speculatively executed”.  A new architecture, such as RISC-V can design more refined mechanisms.  Finally, the API for this defense ought to be designed so as to provide a general mechanism for storage of sensitive assets in “a secure location” which can be non-cacheable memory, a TPM, a programmed flash device, or something else.

Meltdown/Spectre and Implications for OS Security

This week has seen the disclosure of a devastating vulnerability: meltdown and its sister vulnerability, spectre.  Both are a symptom of a larger problem that has evidently gone unnoticed for the better part of 50 years of computer architecture: the potential for side-channels in processor pipeline designs.

Aside: I will confess that I am rather ashamed that I didn’t notice this, despite having a background in all the knowledge areas that should have revealed it.  It took me all of three sentences of the paper before I realized what was going on.  Then again, this somehow got by every other computer scientist for almost half a century.  The conclusion seems to be that we are, all of us, terrible at our jobs…

A New Class of Attack, and Implications

I won’t go in to the details of the attacks themselves; there are already plenty of good descriptions of that, not the least of which is the paper.  My purpose here is to analyze the broader implications with particular focus on the operating systems security perspective.

To be absolutely clear, this is a very serious class of vulnerabilities.  It demands immediate and serious attention from both hardware architects and OS developers, not to mention people further up the stack

The most haunting thing about this disclosure is that it suggests the existence of an entire new class of attack, of which the meltdown/spectre attacks are merely the most obvious.  The problem, which has evidently been lurking in processor architecture design for half a century has to do with two central techniques in processor design:

  1. The fact that processors tend to enhance performance by making commonly-executed things faster.  This is done by a whole host of features: memory caches, trace caches, and branch predictors, to name the more common techniques.
  2. The fact that processor design has followed a general paradigm of altering the order in which operations are executed, either by overlapping multiple instructions (pipelining), executing multiple instructions in parallel (superscalar), executing them out of order and then reconciling the results (out-of-order), and executing multiple possible outcomes and keeping only one result (speculative).

The security community is quite familiar with the impacts of (1): it is the basis for a host of side-channel vulnerabilities.  Much of the work in computer architecture deals with implementing the techniques in (2) while maintaining the illusion that everything is executed one instruction at a time.  CPUs are carefully designed for this with regard to processor state according to the ISA, carefully tracking which operations depend on which and keeping track of the multiple possible worlds that arise from speculation.  What evidently got by us is that the side-channels provided by (1) provide an attacker with the ability to violate the illusion of executing one instruction at a time.

What this almost certainly means is that this is not the last attack of this kind we will see.  Out-of-order speculative execution happens to be the most straightforward to attack.  However, I can sketch out ways in which even in-order pipelines could potentially be vulnerable.  When we consider more obscure architectural features such as trace-caches, branch predictors, instruction fusion/fission, and translation units[0], it becomes almost a certainty that we will see new attacks in this family published well into the future.

A complicating factor in this is that CPU pipelines are extraordinarily complicated, particularly these days.  (I recall a tech talk I saw about the SPARC processor, and its scope was easily as large as the FreeBSD core kernel, or the HotSpot virtual machine.)  Moreover, these designs are typically closed.  There are a lot of different ways a core can be designed, even for a simple ISA, and we can expect entire classes of these design decisions to be vulnerable.

 

[0]: The x86 architecture tends to have translation units that are significantly more complex than the cores themselves, and are likely to be a nest of side-channels.

OS Security

The implications of this for OS (and even application) security are rather dismal.  The one saving virtue in this is that these attacks are read-only: they can be used to exfiltrate data from any memory immediately accessible to the pipeline, but it’s a pretty safe assumption that they cannot be used to write to memory.

That boon aside, this is devastating to OS security.  As a kernel developer, there aren’t any absolute defenses that we can implement without a complete overhaul of the entire paradigm of modern operating systems and major revisions to the POSIX specification.  (Even then, it’s not certain that this would work, as we can’t know how the processors implement things under the hood.)

Therefore, OS developers are left with a choice among partial solutions.  We can make the attacks harder, but we cannot stop them in all cases.  The only perfect defense is to replace the hardware, and hope nobody discovers a new side-channel attack.

Attack Surface and Vulnerable Assets

The primary function of these kinds of attacks is to exfiltrate data across various isolation boundaries.  The following is the most general description of the capabilities of the attack, as far as I can tell:

  1. Any data that can be loaded into a CPU register can potentially be converted into the execution of some number of events before a fault is detected
  2. These execution patterns can affect the performance of future operations, giving rise to a side-channel
  3. These side-channels can be read through various means (note that this need not be done by the same process)

This gives rise to the ability to violate various isolation boundaries (though only for the purposes of observing state):

  • Reads of kernel/hypervisor memory from a guest (aka “meltdown”)
  • Reads of another process’ address space (aka “spectre”)
  • Reads across intra-process isolation mechanisms, such as different VM instances (this attack is not named, but constitutes, among other things, a perfect XSS attack)

A salient feature of these attacks is that they are somewhat slow: as currently described, the attack will incur 2n + 1 cache faults to read out n bits (not counting setup costs).  I do not expect this to last, however.

The most significant danger of this attack is the exfiltration of secret data, ideally small and long-lived data objects.  Examples of key assets include:

  • Master keys for disk encryption (example: the FreeBSD GELI disk encryption scheme)
  • TLS session keys
  • Kerberos tickets
  • Cached passwords
  • Keyboard input buffers, or textual input buffers

More transient information is also vulnerable, though there is an attack window.  The most vulnerable assets are those keys which necessarily must remain in memory and unencrypted for long periods of time.  The best example of this I can think of is a disk encryption key.

Imperfect OS Defense Mechanisms

Unfortunately, operating systems are limited in their ability to adequately defend against these attacks, and moreover, many of these mechanisms incur a performance penalty.

Separate Kernel and User Page Tables

The solution adopted by the Linux KAISER patches is to unmap most of the kernel from user space, keeping only essential CPU metadata and the code to switch page tables mapped.  Unfortunately, this requires a TLB flush for every system call, incurring a rather severe performance penalty.  Additionally, this only protects from access to a kernel address; it cannot stop accesses to other process address spaces or crossing isolation boundaries within a process.

Make Access Attempts to Kernel Memory Non-Recoverable

An idea I proposed on the FreeBSD mailing lists is to cause attempted memory accesses to a kernel address range result in an immediate cache flush followed by non-recoverable process termination.  This avoids the cost of separate kernel and user page tables, but is not a perfect defense in that there is a small window of time in which the attack can be carried out.

Special Handling of Sensitive Assets

Another potential middle-ground is to handle sensitive kernel assets specially, storing them in a designated location in memory (or better yet, outside of main memory).  If a main-memory store is used, this range alone can be mapped and unmapped when crossing into kernel space, thus avoiding most of the overhead of a TLB flush.  This would only protect assets that are stored in this manner; however.  Anything else (most notably the stacks for kernel threads) would remain vulnerable.

Userland Solutions

Userland programs are even less able to defend against such attacks than the kernel; however, there are architectural considerations that can be made.

Avoid Holding Sensitive Information for Long Periods

As with kernel-space sensitive assets, one mitigation mechanism is to avoid retaining sensitive assets for long periods of time.  An example of this is GPG, which keeps the users’ keys decrypted only for a short period of time (usually 5 minutes) after they request to use a key.  While this is not perfect, it limits attacks to a brief window, and presents users with the ability create practices which ensure that they shut off other means of attack during this window.

Minimize Potential for Remote Execution

While this only works on systems that are effectively single-user, minimizing the degree to which remote code can execute on ones system closes many vulnerabilities.  This is obviously difficult in the modern world, where most websites pull in Javascript from dozens if not hundreds of sources, but it can be done with browser plugins like noscript.

JIT/Interpreter Mitigations

As JIT compilers and interpreters are a common mechanism for limited execution of remote code, they are in a position to attempt to mitigate many of these attacks (there is an LLVM patch out to do this).  This is once again an imperfect defense, as they are on the wrong side of Rice’s theorem.  There is no general mechanism for detecting these kinds of attacks, particularly where multiple threads of execution are involved.

Interim Hardware Mitigations

The only real solution is to wait for hardware updates which fix the vulnerabilities.  This take time, however, and we would like some interim solution.

One possible mitigation strategy is to store and process sensitive assets outside of the main memory.  HSMs and TPMs are an example of this (though not one I’m apt to trust), and the growth of the open hardware movement offers additional options.  One in particular- which I may put into effect myself -uses a small FPGA device (such as the PicoEVB) programmed with an open design as such a device.

More generally, I believe this incident highlights the value of these sorts of hardware solutions, and I hope to see increased interest in developing open implementations in the future.

Summary

The meltdown and spectre attacks- severe as they are by themselves -represent the beginning of something much larger.  A new class of attack has come to the forefront, which enables data exfiltration in spite of hardware security mechanisms, and we can and should expect to see more vulnerabilities of this kind in the future.  What makes these vulnerabilities so dangerous is that they directly compromise hardware protection mechanisms on which OS security depends, and thus there is no perfect defense at the OS level against them.

What can and should be done is to adapt and rethink security approaches at all levels.  At the hardware level, a paradigm shift is necessary to avoid vulnerabilities of this kind, as is the consideration that hardware security mechanisms are not as absolute as has been assumed.  At the OS level, architectural changes and new approaches can harden an OS against these vulnerabilities, but they generally cannot repel all attacks.  At the user level, similar architectural changes as well as minimization of attack surfaces can likewise reduce the likelihood and ease of attack, but cannot repel all such attacks.

As a final note, the trend in information security has been increasing focus on application, and particularly web application security.  This event, as well as the fact that a relatively simple vulnerability managed to evade detection by the entire field of computer science for half a century strongly suggests that systems security is not at all the solved problem it is commonly assumed to be, and that new thinking and new approaches are needed in this area.

Design of a Trust System for FreeBSD

About a month ago, I started a discussion on freebsd-hackers and freebsd-security about a system for signed executables, with a focus on signed kernels and kernel modules.  This is part of a larger agenda of mine to equip FreeBSD with OS-level tamper resistance features.

While the initial use of this is for signing the kernel and its modules, and checking signatures during the loader process as well as at runtime when kernel modules are loaded.  However, it is desirable to build a system that is capable of growing in likely directions, such as executable and library signing.

This article details the current state of the design of this system.

Desiderata

I originally outlined a number of goals for this system:

  1. Be able to check for a correct cryptographic signature for any kernel or modules loaded at boot time for some platforms (EFI at a minimum)
  2. Be able to check for a correct cryptographic signature for any kernel module loaded during normal operations (whether or not to do this could be controlled by a sysctl, securelevel, or some similar mechanism)
  3. Work with what’s in the base system already and minimize new additions (ideally, just a small utility to sign executables)
  4. Minimize administrative overhead and ideally, require no changes at all to maintain signed kernel/modules
  5. Have a clear path for supporting signed executables/libraries.
  6. The design must support the case where a system builds locally and uses its own key(s) for signing kernels and modules (and anything else) and must allow the administrator complete control over which key(s) are valid for a given system (ie. no “master keys” controlled by central organizations)
  7. The design must allow for the adoption of new ciphers (there is an inevitable shift to post-quantum ciphers coming in the near future)

I also specified a number of non-goals:

  • Hardware/firmware-based attacks are considered out-of-scope (there is no viable method for defending against them at the OS level)
  • Boot platforms that don’t provide their own signature-checking framework up to loader/kernel can’t be properly secured, and are considered out-of-scope
  • Boot platforms that impose size restrictions prohibiting incorporation of RSA and ED25519 crypto code (ex. i386 BIOS) are considered out-of-scope
  • GRUB support is desirable, however it is not necessary to support GRUB out-of-the-box (meaning a design requiring reasonable modifications to GRUB is acceptable

Considerations

There are several considerations that should weigh in on the design.

FreeBSD Base System

Unlike linux, FreeBSD has a base operating system which contains a number of tools and libraries which provide a set of operating system utilities.  Most notably, the base system contains the OpenSSL (or in some cases, LibreSSL) crypto suite.  This includes an encryption library as well as tools capable of creating and managing key-pairs and other cryptographic data in a variety of formats.

Additionally, the FreeBSD base system contains libelf, which is a library that provides mechanisms for manipulating ELF binaries.  Additionally, the base system provides the binutils suite, including objcopy, which are command-line tools capable of manipulating ELF binaries.

Note that only some of these components (namely the signelf tool) exist at the present; the rest of the components exist only as man pages that describe them at present.

Public-Key Cryptography

The FreeBSD kernel does not currently incorporate code for public-key cryptography, and direct incorporation of OpenSSL into the kernel has proven infeasible.  Additionally, parsing code needs to be incorporated into the kernel for any formats that are used.  Options here include incorporation of code from the NaCl library, which provides a very lightweight implementation of both RSA 4096 and Ed25519, as well as creating a minimal library out of code harvested from OpenSSL or LibreSSL.

A note on elliptic curve cryptography: the state of support for safe elliptic curves is sad.  In my drafts of the man pages, I have mandated that the only acceptable curves are those that satisfy the security properties described by the SafeCurves project.  At this time, these include M-221, E-222, Curve1174, Curve25519, E-382, M-383, Curve383187, Curve41417, Goldilocks-448, M-511, and E-521.  Unfortunately, none of these is supported by OpenSSL at this time, though Curve25519 support is supposedly coming soon.  However, I would prefer to write specs that mandate the right curves (and thus put pressure on crypto libraries) than cave to using bad ones.

Modifications to GRUB

GRUB provides the best option for FreeBSD coreboot support at this time.  It also provides an existing mechanism for signing binaries.  However, this mechanism is deficient in two ways.  First, it relies on external signatures, which would complicate administration and require modification of virtually all installer programs, as well as run the risk of stale signatures.  Second, it relies on the gnupg toolset, which is not part of the FreeBSD base system.  Thus, it is inevitable that GRUB will need to be patched to support the signed executables proposed by this design.  However, we should make efforts to keep the necessary changes as minimal as possible.

Signing and Trust System Design

The signing and trust system consists of a number of components, some of which are standards, some of which are interfaces, and some of which are tools.  The core feature, of course, is the signed ELF convention.  The signelf tool provides a one-stop tool for signing large numbers of executables.  The trust system provides a system-level mechanism for registering and maintaining verification keys that are used to check signatures on kernel modules.  Finally, the portable verification library provides a self-contained code package that can be dropped into the kernel, the loader, or a third-party codebase like GRUB.

Note that this design is not yet implemented, so it may be subject to change.  Also, it has not yet undergone review on the FreeBSD lists, so it should be considered more of a proposal.

Signed ELF Binaries

The ELF format is very flexible, and provides a generic mechanism for storing metadata.  The signed ELF convention utilizes this to store signatures in a special section within the binary itself.  A signed ELF binary contains a section named .sign, which contains a detached PKCS#7 signature in DER encoding for the file.  This signature is computed (and checked) on the entire file, with the .sign section itself being replaced by zero data of equal size and position.

Signing an ELF binary is somewhat involved, as it requires determining the size of a signature, creating a new section (along with its name), recomputing the ELF layout, computing the signature, and writing it into the section.  Checking a signature is considerably simpler: it involves merely copying the signature, overwriting the .sign section with zeros, and then checking the signature against the  entire file.

The PKCS#7 format was chosen because it is an established standard which supports detached signatures as well as many other kinds of data.  The signatures generated for signed ELF files are minimal and do not contain certificates, attributes, or other data (a signature for RSA-4096 is under 800 bytes); however, the format is extensible enough to embed other data, allowing for future extensions.

The signelf Tool

Signed ELF binaries can be created and checked by adroit usage of the objcopy and openssl command-line tools.  This is quite tedious, however.  Moreover, there are certain use cases that are desirable, like signing a batch of executables using an ephemeral key, discarding the key, and generating a certificate for verification.  The signelf tool is designed to be a simplified mechanism for signing batches of executables which provides this additional functionality.  It is a fairly straightforward use of libelf and OpenSSL, and should be able to handle the binaries produced by normal compilation.  Additionally, the signelf tool can verify signed ELF files.  The signelf code is currently complete, and works on a kernel as well as modules.

The Trust System

In order to check signatures on kernel modules (and anything else), it is necessary to establish and maintain a set of trusted verification keys in the kernel (as well as in the boot loader).  In order for this system to be truly secure, at least one trust root key must be built into the kernel and/or the boot loader, which can then be used to verify other keys.  The trust system refers to the combination of kernel interfaces, standard file locations, and conventions that manage this.

System Trust Keys and Signing Keys

The (public) verification keys used to check signatures as well as the (private) signing keys used to generate signatures are kept in the /etc/trust/ directory.  Verification keys are stored in /etc/trust/certs, in the X509 certificate format, and private keys are stored in /etc/trust/keys in the private key format.  Both are stored in the PEM encoding (as is standard with many OpenSSL applications).

There is no requirement as to the number, identity, or composition of verification or signing keys.  Specifically, there is not and will never be any kind of mandate for any kind of verification key not controlled by the owner of the machine.  The trust system is designed to be flexible enough to accommodate a wide variety of uses, from machines that only trust executables built locally, to ones that trust executables built on an in-house machine only, to those that trust executables built by a third party (such as the FreeBSD foundation), or any combination thereof.

The preferred convention, however, is to maintain a single, per-machine keypair which is then used to sign any additional verification keys.  This keypair should be generated locally for each machine, and never exported from the machine.

Trust Keys Library

Keys under /etc/trust/certs will be converted into C code constants and subsequently compiled into a static library providing the raw binary data for the keys during the buildworld process.  This provides the mechanism for building keys into the kernel, loader, and other components.  These keys are known as trust root keys, as they provide the root set for all trusted keys.

Kernel Trust Interface

The kernel trust interface provides access to the set of verification keys trusted by the kernel.  This consists of an in-kernel interface as well as a user-facing device interface.  The in-kernel interface looks like an ordinary key management system (KMS) interface.  The device interface provides two primary mechanisms: access to the current set of trusted keys and the ability to register new keys or revoke existing ones.

Access to the existing database is accomplished through a read-only device node which simply outputs all of the existing trusted keys in PEM-encoded X509 format.  This formatting allows many OpenSSL applications to use the device node itself as a CA root file.  Updating the key database is accomplished by writing to a second device node.  Writing an X509 certificate signed by one of the existing trusted keys to this device node will cause the key contained in the certificate to be added to the trusted key set.  Writing a certificate revocation list (CRL) signed by a trusted key to the device node will revoke the keys in the revocation list as well as any keys whose signature chains depend on them.  Trust root keys cannot be revoked, however.

This maintains the trusted key set in a state where any trusted key has a signature chain back to a trust root key.

Portable Verification Library

The final piece of the system is the portable verification library.  This library should resemble a minimal OpenSSL-like API that performs parsing/encoding of the necessary formats (PKCS#7, X509, CRL), or a reduced subset thereof and public-key signature verification.  I have not yet decided whether to create this from harvesting code from OpenSSL/LibreSSL or write it from scratch (with code from NaCl), but I’m leaning toward harvesting code from LibreSSL.

Operation

The trust system performs two significant roles in the system as planned, and can be expanded to do more things in the future.  First, it ensures that loader only loads kernels and modules that are signed.  Second, it can serve as a kind of system-wide keyring (hence the device node that looks like a typical PEM-encoded CAroot file for OpenSSL applications).  The following is an overview of how it would operate in practice.

Signature Checking in the loader

In an EFI environment, boot1.efi and loader.efi have a chain of custody provided by the EFI secure boot framework.  This is maintained from boot1.efi to loader.efi, because of the use of the EFI loaded image interface.  The continuation of the chain of custody must be enforced directly by loader.efi.  To accomplish this, loader will link against the trust key library at build time to establish root keys.  These in turn can either be used to check the kernel and modules directly, or they can be used to check a per-kernel key (the second method is recommended; see below).

Per-Kernel Ephemeral Keys

The signelf utility was designed with the typical kernel build process in mind.  The kernel and all of its modules reside in a single directory; it’s a simple enough thing to run signelf on all of them as the final build step.  Additionally, signelf can generate an ephemeral key for signing and write out the verification certificate after it finishes.

This gives rise to a use pattern where every kernel is signed with an ephemeral key, and a verification certificate is written into the kernel directory.  This certificate is in turn signed by the local trust root key (signelf does this as part of the ephemeral key procedure).  In this case, the loader first attempts to load the verification certificate for a kernel, then it loads the kernel and all modules.

Signed Configuration Files

The FreeBSD loader relies on several files such as loader.4th, loader.conf, loader.menu, and others that control its behavior in significant ways.  Additionally, one can foresee applications of this system that rely on non-ELF configuration files.  For loader, the simplest solution is to store these files as non-detached PKCS#7 messages (meaning, the message and file contents are stored together).  Thus, loader would look for loader.conf.pk7, loader.4th.pk7, and so on.  A loader built for secure boot would look specifically for the .pk7 files, and would require signature verification in order to load them.

The keybuf Interface

The kernel keybuf interface was added in a patch I contributed in late March 2017.  It is used by GELI boot support to pass keys from the boot phases to the kernel.  However, it was designed to support up to 64 distinct 4096-bit keys without modification; thus it can be used with RSA-4096.  An alternative to linking the trust key library directly into the kernel is to have it receive the trusted root key as a keybuf entry.

This approach has advantages and disadvantages.  The advantage is it allows a generic kernel to be deployed to a large number of machines without rebuilding for each machine.  Specifically, this would allow the FreeBSD foundation to publish a kernel which can make use of local trust root keys.  The primary disadvantage is that the trust root keys are not part of the kernel and thus not guaranteed by the signature checking.  The likely solution will be to support both possibilities as build options.

Key Management

The preferred scheme for trust root keys is to have a local keypair generated on each machine, with the local verification certificate serving as the sole trust root key.  Any vendor keys that might be used would be signed by this keypair and loaded as intermediate keys.  Every kernel build would produce an ephemeral key which would be signed by the local keypair.  Kernel builds originating from an organization would also be signed by an ephemeral key, whose certificate is signed by the organization’s keypair.  For example, the FreeBSD foundation might maintain a signing key, which it uses to sign the ephemeral keys of all kernel builds it publishes.  An internal IT organization might do the same.

It would be up to the owner of a machine whether or not to trust the vendor keys originating from a given organization.  If the keys are trusted, then they are signed by the local keypair.  However, it is always an option to forego all vendor keys and only trust locally-built kernels.

An alternate use might be to have no local signing key, and only use an organizational trust root key.  This pattern is suitable for large IT organizations that produce lots of identical machines off of a standard image.

Conclusion

This design for the trust system and kernel/module signing is a comprehensive system-wide public-key trust management system for FreeBSD.  Its initial purpose is managing a set of keys that are used to verify kernels and kernel modules.  However, the system is designed to address the issues associated with trusted key management in a comprehensive and thorough way, and to leave the door open to many possible uses in the future.