Forwarded: Re: [syzbot] [gfs2?] KASAN: slab-use-after-free Write in gfs2_qd_dealloc (3)

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Subject: Re: [syzbot] [gfs2?] KASAN: slab-use-after-free Write in gfs2_qd_dealloc (3)
Author: tristmd@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

>From bcb810e8bbe744753bc43fd4f3cb5946a3ae91ee Mon Sep 17 00:00:00 2001
From: Tristan Madani <tristan@talencesecurity.com>
Date: Thu, 30 Apr 2026 11:05:04 +0000
Subject: [PATCH] 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.

Fixes: a475c5dd16e5 ("gfs2: Free quota data objects synchronously")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
 fs/gfs2/super.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
index a2ea121331f18..4d854556b5299 100644
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -643,6 +643,7 @@ static void gfs2_put_super(struct super_block *sb)
 	gfs2_delete_debugfs_file(sdp);
 
 	gfs2_sys_fs_del(sdp);
+	rcu_barrier();
 	free_sbd(sdp);
 }
 
-- 
2.47.3