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Hardening against malicious stateful data

Motivation

Chrome OS has Verified Boot, which is designed to make sure that the system will only run binaries that are trusted, starting at firmware and continuing all the way through the boot process to Chrome. It turns out this is not sufficient, as the data dependencies of code running during boot pull unverified data from the stateful partition. This may be configuration data, device state indicators, data caches etc. It turns out that data dependencies in the boot process pose a security risk: Attackers that have a root exploit control the stateful partition, allowing them to stage malicious file system state (file contents, symlinks, directory layout, etc.) that will affect a subsequent boot in a way that re-exploits the device, thus providing a vehicle for the attacker to get a persistent exploit.

Both examples of persistent Chrome OS exploits that have been reported to us up to now exploit this (symlink attacks) as their persistence mechanism. Thus, we need to:

  1. Eliminate data dependencies on stateful data as much as possible from the boot process.

  2. Mitigate any legit and required dependencies to move the bar to a point where exploiting them becomes really hard, if not impossible.

This document describes approaches to achieving these goals..

Anatomy of a persistent attack

Here's a quick summary of the usual steps involving a persistent exploit. This is useful for reference when assessing mitigations:

  1. Attacker exploits the running system. This might be in the form of Chrome exploit, a system daemon exploit, a full root exploit, or even a kernel exploit.

  2. Attacker manipulates state carried over across reboots. Depending on what privileges the attacker managed to acquire, they have different options here. A full root or kernel exploit will allow to make arbitrary changes to the stateful file system for example, as well as other stateful storage such as TPM and VPD. If the attacker has "only" a Chrome exploit, they may still manipulate the stateful file system subtrees that are writeable by user chronos. Similarly, if the attacker controls a system daemon, they can manipulate that process' state (assuming the system daemon is running within a sandbox). Some approaches to manipulation:

    1. Store malicious data in a file that is read and parsed, interpreted etc. after reboot.

    2. Manipulate the file system layout: Insert symlinks or hard links that will redirect write actions during boot (such as mkdir, writing cache files etc.).

    3. Manipulate file system state to trigger unintended execution paths (example: make files unreadable / wrong type to trigger untested/inappropriate error handling or fallback behavior).

  3. System reboots.

  4. Re-gaining code execution: Verified boot ensures that kernel and root file system remain intact, i.e. the attacker can't change code there to re-acquire control of the system. Instead, the attacker will have to arrange for the regular boot flow to "take a wrong turn", i.e. trick legit code running during boot to perform inadvertent actions that will give the attacker code execution again. This is where the manipulated stateful data comes in: It will be consumed by init scripts and system services and may affect their behavior in a variety of ways. Some of the more obvious are:

    1. Trigger execution of shell scripts stored on stateful.

    2. Exploiting weaknesses in config parsing to gain code execution within a system service.

    3. Manipulating user-installed code that the system will run automatically, such as extensions installed in a user profile.

Mitigations

The remainder of the document discusses hardening measures that will prevent certain classes of malicious stateful data to have an adverse effect. Note that there is no comprehensive solution - any useful device will have to store some data somewhere (in the cloud if not locally) and users expect to be able to see their previous state after a reboot. This inherently implies that we need to carry over state, so some risk will always remain. It makes sense to prioritize mitigation work to reduce impact of successful exploits. To that effect, we should start with addressing stateful data dependencies within highly-privileged code (such as init scripts, system daemons), then work our way towards less privileged code.

Restricting symlink traversal

Both cases of persistent Chrome OS exploits that have been reported via the Chrome vulnerability reward program relied heavily on placing symlinks on the stateful file system to re-exploit the system after a reboot. The idea is to place a symlink somewhere on a path written to by a privileged process during boot to redirect the write to a different location. In some cases, the data that gets written can also be controlled by the attacker, e.g. when the code in question is copying files from one location in the stateful file system to another. It turns out that intentional usage of symlinks is actually very rare on the stateful file system. Given that and the relative success of symlink attacks, it makes sense to generally disallow symlink traversal.

To prevent symlinks to be traversed on the stateful file system, there are a variety of possible approaches:
  1. Change all code that accesses files to double-check it didn't follow a symlink inadvertently. Note that the O_NOFOLLOW open flag and symlink-aware version of system calls (such as lstat() in favor of stat()) are not sufficient, since they only cover the last path component. Symlinks in parent components are still followed. A working way of implementing a symlink traversal check is via readlink() on /proc/self/fd/N. Unfortunately, it's just not practical to do this for all file access in shell scripts, 3rd-party software etc., so while this approach is technically feasible, it doesn't fly in practice.
  2. Add a mount option in a kernel patch, so symlink traversal can be disabled on a per-mount basis. The BSDs have nosymfollow which implements this. We have proposed the same solution for Linux upstream, but it was met with skepticism. Reality is that few Linux distributions be able to use this meaningfully (it only makes sense when you have a verified or at least read-only rootfs anyways). We could carry the patch in the Chrome OS kernel tree indefinitely, but that'll cause maintenance overhead as file system internals change over time.
  3. Scrub symlinks after mount. This would require making a full pass over the mounted file system and remove any unintended symlinks. This will adversely affect boot times since we'd have to do this before starting any init jobs that read stateful data.
  4. Use SELinux to apply policy to prevent symlink traversal. This would require the entire stateful file system to be re-labelled after mount to make sure there aren't any labels present that would allow symlink traversal in locations that shouldn't do so. This approach suffers from the same boot time issue as the previous one.
  5. Reject symlinks via the LSM inode_follow_link hook in the Chromium OS LSM. Implement the logic in a non-invasive way in the kernel to keep maintenance overhead low.
After considering the pros and cons of the approaches listed above, we've chosen to go with the LSM inode_follow_link approach. Design highlights:
  • The inode_follow_link hook can look at the dentry and inode of the traversed symlink location. We thus need to hook up any information require to make a decision in such a way that it is accessible from the inode/dentry. Note that we can't simply fail all symlink traversal, since there are important cases of legit symlink usage on the root file system, e.g. the shared library symlinks in /usr/lib. We also want to be able to selectively allow symlinks for specific locations even though symlink traversal should generally be blocked on the stateful file system.
  • To achieve per-inode decisions, the code uses fsnotify infrastructure, which allows to attach "inode marks" to an inode. This is also used by other kernel subsystems (e.g. the audit code) and allows callbacks to be invoked when something happens. We only rely on the notification send on inode destruction so we can clean up tracked state, but we can use the inode mark to hold symlink traversal policy for the inode.
  • Symlink traversal policy is evaluated for a path in question by searching up the parent directory chain until an inode is found that carries a specific traversal policy. This allows setting the policy to block symlinks on the root inode, which will thus be inherited by the entire file system. This also allows exceptions to be granted by setting the policy to allow traversal for a specific directory, which will grant an exception for the subtree rooted at the directory.
  • Symlink traversal policy can be configured via userspace via securityfs.
  • Whenever an attempt at traversing a symlink runs into a policy that blocks the access, the system call wiil fail with EPERM and a kernel warning will get logged.
  • The kernel warning collector uploads symlink traversal blocking warnings via the crash reporter, so we have some insight whether devices hit restrictions inadvertently.
  • The code uses only "public" kernel APIs, i.e. it is entirely contained within the LSM module. No kernel changes elsewhere are needed. This keeps maintenance overhead at the absolute minimum.
To make use of the symlink traversal policy mechanism provided by the LSM, we'll require a few userspace changes. chromeos_startup will be responsible to configure symlink traversal to be blocked on the stateful and encrypted stateful file systems. We'll need to allow a few exceptions:
  • /var/cache/vpd, /var/cache/echo: These directories contain backwards compatibility symlinks set up by dump_vpd_log. We should ideally clean up consumers of these symlinks and remove the exception.
  • /var/lib/timezone: This directory contains a symlink to the actual time zone file and is maintained by Chrome. This is a low-risk exception as this path is generally only sees read-only access by privileged code.
  • /var/log: The log directory contains a number of convenience symlinks that point at the "latest" log file for ui, chrome, power_manager etc. logs. This is somewhat higher risk since privileged system daemons write to files in /var/log. We should consider restricting this further, but will grant an exception for now.
  • /home: When using ext4 encryption, encrypted user data actually resides within the stateful file system and encryption is handled on a per-file basis. To avoid blowing up scope considerably for initial roll-out, we'll whitelist /home so user data maintained by Chrome can still use symlinks. Note that this is low risk for the most part since the majority of the access are happening within chronos user context. There is some data that is access by privileged daemons in the /home/chronos/root/ bind-mounts though. We should eventually lock down symlink traversal either for the entire user data subtree, or at least for the locations access by system daemons.
  • /mnt/stateful_partition/dev_image: This directory contains utilties that are convenient for developers running in dev mode. We'll grant an exception for this when setting up this location for developer mode.
Further exceptions can be added in justified cases.
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