On 12/27/2025 10:49 AM, Szymon Wilczek wrote:
Syzbot reported a slab-use-after-free issue in f2fs_write_end_io():
BUG: KASAN: slab-use-after-free in f2fs_write_end_io+0x9b9/0xb60
Read of size 4 at addr ffff88804357d170 by task kworker/u4:4/45
The race condition occurs between the filesystem unmount path
(kill_f2fs_super) and the asynchronous I/O completion handler
(f2fs_write_end_io).
When unmounting, kill_f2fs_super() frees the sbi structure. However, if
there are pending CP_DATA writes, the f2fs_write_end_io() callback might
still be running in softirq context and attempt to access sbi->cp_wait,
causing a use-after-free.
Fix this by calling synchronize_rcu() after f2fs_wait_on_all_pages()
but before kfree(sbi). Since bio completion callbacks run in softirq
context, which is an implicit RCU read-side critical section,
synchronize_rcu() ensures all in-flight callbacks have completed
before we free sbi.
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=b4444e3c972a7a124187
Signed-off-by: Szymon Wilczek <[email protected]>
---
v3: Simplified to minimal fix - only super.c change with synchronize_rcu(),
as pointed out by Chao Yu that data.c changes are not necessary since
synchronize_rcu() alone guarantees sbi won't be freed before callbacks
complete.
v2: Add synchronize_rcu() to wait for softirq bio callbacks to complete.
---
fs/f2fs/super.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index c4c225e09dc4..924bc30d08b6 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -5454,6 +5454,8 @@ static void kill_f2fs_super(struct super_block *sb)
kill_block_super(sb);
/* Release block devices last, after fscrypt_destroy_keyring(). */
if (sbi) {
+ f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA);
f2fs_wait_on_all_pages() -> f2fs_submit_merged_write() will access sbi->write_io
which should has been released in f2fs_put_super()?
Thanks,
+ synchronize_rcu();
destroy_device_list(sbi);
kfree(sbi);
sb->s_fs_info = NULL;
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