[PATCH] erofs: fix use-after-free on sbi->sync_decompress

[Gao Xiang]

z_erofs_decompress_kickoff() can race with filesystem unmount, causing
a use-after-free on sbi->sync_decompress.

When I/O completes, z_erofs_endio() calls z_erofs_decompress_kickoff()
to queue z_erofs_decompressqueue_work() asynchronously. Then, after all
folios are unlocked, unmount workflow can proceed and sbi will be freed
before accessing to sbi->sync_decompress.

Thread (unmount)        I/O completion        kworker
                        queue_work
                                              z_erofs_decompressqueue_work
                                               (all folios are unlocked)
cleanup_mnt
 ..
 erofs_kill_sb
  erofs_sb_free
   kfree(sbi)
                        access sbi->sync_decompress  // UAF!!

Fixes: 40452ffca3c1 ("erofs: add sysfs node to control sync decompression strategy")
Reported-by: syzbot+52bae5c495dbe261a0bc@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=52bae5c495dbe261a0bc
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
---
 fs/erofs/zdata.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 27ab7bd844ec..c6240dccbb0f 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -1455,6 +1455,9 @@ static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
 	if (atomic_add_return(bios, &io->pending_bios))
 		return;
 	if (z_erofs_in_atomic()) {
+		/* See `sync_decompress` in sysfs-fs-erofs for more details */
+		if (sbi->sync_decompress == EROFS_SYNC_DECOMPRESS_AUTO)
+			sbi->sync_decompress = EROFS_SYNC_DECOMPRESS_FORCE_ON;
 #ifdef CONFIG_EROFS_FS_PCPU_KTHREAD
 		struct kthread_worker *worker;
 
@@ -1471,9 +1474,6 @@ static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
 #else
 		queue_work(z_erofs_workqueue, &io->u.work);
 #endif
-		/* See `sync_decompress` in sysfs-fs-erofs for more details */
-		if (sbi->sync_decompress == EROFS_SYNC_DECOMPRESS_AUTO)
-			sbi->sync_decompress = EROFS_SYNC_DECOMPRESS_FORCE_ON;
 		return;
 	}
 	gfp_flag = memalloc_noio_save();
-- 
2.43.5