CVE-2026-63804

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

gfs2: fix use-after-free in gfs2_qd_dealloc

gfs2_qd_dealloc(), called as an RCU callback from gfs2_qd_dispose(),

accesses the superblock object sdp through qd->qd_sbd after freeing qd.

It does so to decrement sd_quota_count and wake up sd_kill_wait.

However, by the time the RCU callback runs, gfs2_put_super() may have

already freed sdp via free_sbd(). This can happen when

gfs2_quota_cleanup() is called during unmount: it disposes of quota

objects via call_rcu() and then waits on sd_kill_wait with a 60-second

timeout. If the timeout expires, or if gfs2_gl_hash_clear() triggers

additional qd_put() calls that schedule more RCU callbacks after the

wait completes, gfs2_put_super() will proceed to free the superblock

while RCU callbacks referencing it are still pending.

Add an rcu_barrier() before free_sbd() in gfs2_put_super() to ensure

all pending RCU callbacks (including gfs2_qd_dealloc) have completed

before the superblock is freed.