[PATCH] gfs2: keep glocks alive while AIL entries exist

[Eduard Zateev <hackerowskiy@gmail.com>]

A buffer on a glock AIL list can outlive the glock reference held by its
inode. During withdrawal, gfs2_ail_drain() later removes the buffer from
gl_ail_list, which dereferences the freed glock.

Hold one glock reference while its AIL count is nonzero. Acquire the
reference for the first AIL entry and release it asynchronously when the
last entry is removed, since AIL removal runs under sd_ail_lock.

Build-tested: x86-64 GFS2 objects with W=1; arm64 GFS2 objects
also compile.
The syzbot report has no usable reproducer on current HEAD, so a runtime
test was not possible.

Fixes: 864174f97647 ("gfs2: Don't cache unreferenced glocks")
Reported-by: syzbot+ab91b239b20ae188b9cb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ab91b239b20ae188b9cb
Assisted-by: LLM
Signed-off-by: Eduard Zateev <hackerowskiy@gmail.com>
---
 fs/gfs2/log.c  | 5 ++++-
 fs/gfs2/lops.c | 3 ++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/gfs2/log.c b/fs/gfs2/log.c
index b55daf1e638..aa5f2a9a94f 100644
--- a/fs/gfs2/log.c
+++ b/fs/gfs2/log.c
@@ -74,10 +74,13 @@ unsigned int gfs2_struct2blk(struct gfs2_sbd *sdp, unsigned int nstruct)
 
 static void gfs2_remove_from_ail(struct gfs2_bufdata *bd)
 {
+	struct gfs2_glock *gl = bd->bd_gl;
+
 	bd->bd_tr = NULL;
 	list_del_init(&bd->bd_ail_st_list);
 	list_del_init(&bd->bd_ail_gl_list);
-	atomic_dec(&bd->bd_gl->gl_ail_count);
+	if (atomic_dec_and_test(&gl->gl_ail_count))
+		gfs2_glock_put_async(gl);
 	brelse(bd->bd_bh);
 }
 
diff --git a/fs/gfs2/lops.c b/fs/gfs2/lops.c
index 341f5bb844b..c4e63fb375b 100644
--- a/fs/gfs2/lops.c
+++ b/fs/gfs2/lops.c
@@ -120,8 +120,9 @@ static void gfs2_unpin(struct gfs2_sbd *sdp, struct buffer_head *bh,
 		brelse(bh);
 	} else {
 		struct gfs2_glock *gl = bd->bd_gl;
+		if (atomic_inc_return(&gl->gl_ail_count) == 1)
+			gfs2_glock_hold(gl);
 		list_add(&bd->bd_ail_gl_list, &gl->gl_ail_list);
-		atomic_inc(&gl->gl_ail_count);
 	}
 	bd->bd_tr = tr;
 	list_add(&bd->bd_ail_st_list, &tr->tr_ail1_list);
-- 
2.55.0