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.