CVE-2026-63804

Description

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.

Predictions

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-63804
• https://git.kernel.org/stable/c/4fe388218826df8607ae41a6305df67db08a9093
• https://git.kernel.org/stable/c/8745d9f7e1682c39f0a1578895ac74205e2a6757
• https://git.kernel.org/stable/c/9d0d5ba20cad661f7f287d4c66d2c19022ce2fd0
• https://git.kernel.org/stable/c/b85ef03f726b15047a6fa6d11b639bdf6c0ee4f0
• https://git.kernel.org/stable/c/f9c9ec2c319f843b70ecdf939d48b52d189bc081
• https://www.suse.com/security/cve/CVE-2026-63804.html