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_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 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 suite's thin-provisioning suites for delete and suspend and no regressions were observed.

This vulnerability is rated as Moderate severity because exploitation requires elevated administrative privileges to configure and tear down device-mapper targets, limiting the attack surface to privileged users or race conditions during storage teardown. While Device Mapper thin provisioning is available in Red Hat Enterprise Linux, the race condition occurs exclusively when a thin-provisioned target is removed while handling concurrent I/O, leading to a kernel crash rather than arbitrary code execution or confidentiality loss. Unprivileged users cannot trigger this race condition under standard system configurations.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.