linux-kernel - [PATCH v3] f2fs: fix use-after-free in f2fs_write_end

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Message-ID: <20251227024944.8612-1-swilczek.lx@gmail.com>
Date: Sat, 27 Dec 2025 03:49:44 +0100
From: Szymon Wilczek <swilczek.lx@...il.com>
To: jaegeuk@...nel.org,
	chao@...nel.org
Cc: linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	syzbot+b4444e3c972a7a124187@...kaller.appspotmail.com,
	Szymon Wilczek <swilczek.lx@...il.com>
Subject: [PATCH v3] f2fs: fix use-after-free in f2fs_write_end_io

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: syzbot+b4444e3c972a7a124187@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b4444e3c972a7a124187
Signed-off-by: Szymon Wilczek <swilczek.lx@...il.com>
---
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);
+		synchronize_rcu();
 		destroy_device_list(sbi);
 		kfree(sbi);
 		sb->s_fs_info = NULL;
-- 
2.52.0