Userlevel heap temporal memory safety (experimental)

CheriBSD 23.11 incorporates support for userlevel heap temporal memory safety based on a load-barrier extension to Cornucopia, which is inspired by garbage-collection techniques. A forthcoming paper at ASPLOS 2024 on the Cornucopia Reloaded technique will describe these differences in detail.

This feature involves a collaboration between the kernel (which provides asynchronous capability revocation with VM acceleration) and the userlevel heap allocator (which quarantines freed memory until revocation of any pointers to it) to ensure that memory cannot be reallocated until there are no outstanding valid capabilities lasting from its previous allocation. The userlevel memory allocator and the kernel revoker share an epoch counter that counts the number of completed atomic revocation sweeps. Memory added to quarantine in one epoch cannot be removed from quarantine until at least one complete epoch has passed -- i.e., the epoch counter has been increased by 2. More information on temporal memory safety support can be found in the mrs(3) man page:

man mrs

Checking whether temporal safety is globally enabled

Use the sysctl(8) command to inspect the value of the security.cheri.runtime_revocation_default system MIB entry:

sysctl security.cheri.runtime_revocation_default

This sysctl sets the default policy for revocation used by processes on startup. We recommend setting this in /boot/loader.conf, which is processed by the boot loader before any user processes start.

Controlling revocation by binary or process

You can forcefully enable or disable revocations for a specific binary or process with elfctl(1) or proccontrol(1) and ignore the default policy:

elfctl -e <+cherirevoke or +nocherirevoke> <binary>

proccontrol -m cherirevoke -s <enable or disable> <program with arguments>

You can read more on these commands in the mrs(3) man page.

Synchronous revocation

Revocation normally occurs asynchronously, with a memory quarantine preventing memory reuse until revocation of any pointers to that memory. You can insert a specific call in your program to await synchronous revocation:

malloc_revoke();

Synchronous revocation on every call to free() can be configured using the _RUNTIME_REVOCATION_EVERY_FREE_ENABLE environmental variable.

Note that synchronous revocation can incur extremely high expense.

Monitoring revocation in processes

Use the procstat cheri -v command to inspect the CHERI memory-safety state of a target process. For example:

# procstat cheri -v 923 1012
  PID COMM                C QUAR  RSTATE                              EPOCH
  923 seatd               P  yes    none                                  0
 1012 Xorg                P  yes    none                               0xd2

Both processes in this example use a pure-capability process environment, have quarantining enabled, and are not currently revoking. seatd has never needed to perform a revocation pass, as it remains in epoch 0, whereas X.org has a non-zero epoch and has performed multiple passes.