CVE-2025-21664

In the Linux kernel, the following vulnerability has been resolved:

dm thin: make get_first_thin use rcu-safe list first function

The documentation in rculist.h explains the absence of list_empty_rcu() and cautions programmers against relying on a list_empty() -> list_first() sequence in RCU safe code. This is because each of these functions performs its own READ_ONCE() of the list head. This can lead to a situation where the list_empty() sees a valid list entry, but the subsequent list_first() sees a different view of list head state after a modification.

In the case of dm-thin, this author had a production box crash from a GP fault in the process_deferred_bios path. This function saw a valid list head in get_first_thin() but when it subsequently dereferenced that and turned it into a thin_c, it got the inside of the struct pool, since the list was now empty and referring to itself. The kernel on which this occurred printed both a warning about a refcount_t being saturated, and a UBSAN error for an out-of-bounds cpuid access in the queued spinlock, prior to the fault itself. When the resulting kdump was examined, it was possible to see another thread patiently waiting in thin_dtrs synchronize_rcu.

The thin_dtr call managed to pull the thin_c out of the active thins list (and have it be the last entry in the active_thins list) at just the wrong moment which lead to this crash.

Fortunately, the fix here is straight forward. Switch get_first_thin() function to use list_first_or_null_rcu() which performs just a single READ_ONCE() and returns NULL if the list is already empty.

This was run against the devicemapper test suites thin-provisioning suites for delete and suspend and no regressions were observed.

Affected Software

References

• https://git.kernel.org/stable/c/12771050b6d059eea096993bf2001da9da9fddff
• https://git.kernel.org/stable/c/6b305e98de0d225ccebfb225730a9f560d28ecb0
• https://git.kernel.org/stable/c/802666a40c71a23542c43a3f87e3a2d0f4e8fe45
• https://git.kernel.org/stable/c/80f130bfad1dab93b95683fc39b87235682b8f72
• https://git.kernel.org/stable/c/cbd0d5ecfa390ac29c5380200147d09c381b2ac6
• https://git.kernel.org/stable/c/cd30a3960433ec2db94b3689752fa3c5df44d649
• https://git.kernel.org/stable/c/ec037fe8c0d0f6140e3d8a49c7b29cb5582160b8