Description

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_dtr's
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 us...

Classification

CVE ID: CVE-2025-21664

Affected Products

Vendor: Linux

Product: Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.04% (probability of being exploited)

EPSS Percentile: 12.61% (scored less or equal to compared to others)

EPSS Date: 2025-02-19 (when was this score calculated)

References

https://git.kernel.org/stable/c/12771050b6d059eea096993bf2001da9da9fddff
https://git.kernel.org/stable/c/6b305e98de0d225ccebfb225730a9f560d28ecb0
https://git.kernel.org/stable/c/cbd0d5ecfa390ac29c5380200147d09c381b2ac6
https://git.kernel.org/stable/c/80f130bfad1dab93b95683fc39b87235682b8f72

Timeline