CVE-2022-50668

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

ext4: fix deadlock due to mbcache entry corruption

When manipulating xattr blocks, we can deadlock infinitely looping

inside ext4_xattr_block_set() where we constantly keep finding xattr

block for reuse in mbcache but we are unable to reuse it because its

reference count is too big. This happens because cache entry for the

xattr block is marked as reusable (e_reusable set) although its

reference count is too big. When this inconsistency happens, this

inconsistent state is kept indefinitely and so ext4_xattr_block_set()

keeps retrying indefinitely.

The inconsistent state is caused by non-atomic update of e_reusable bit.

e_reusable is part of a bitfield and e_reusable update can race with

update of e_referenced bit in the same bitfield resulting in loss of one

of the updates. Fix the problem by using atomic bitops instead.

This bug has been around for many years, but it became *much* easier

to hit after commit 65f8b80053a1 ("ext4: fix race when reusing xattr

blocks").