linux-kernel - [PATCH v2] f2fs: fix use-after-free in f2fs_write_end

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Message-ID: <20251226142024.48837-1-swilczek.lx@gmail.com>
Date: Fri, 26 Dec 2025 15:20:24 +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 v2] 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.

To fix this:

1. In f2fs_write_end_io(), check SBI_IS_CLOSE flag early and skip the
   wake_up() call if the filesystem is shutting down. Move the wake_up
   inside the loop for correct synchronization.

2. In kill_f2fs_super(), after f2fs_wait_on_all_pages() returns (meaning
   the page count is zero), call synchronize_rcu() 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.

The combination of these two changes eliminates the UAF window: the
is_close check provides fast-path optimization (skip wake_up when no
one is waiting), while synchronize_rcu() provides the hard guarantee
that no callback is accessing sbi when we free it.

Reported-by: syzbot+b4444e3c972a7a124187@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b4444e3c972a7a124187
Signed-off-by: Szymon Wilczek <swilczek.lx@...il.com>
---
v2: Add synchronize_rcu() to wait for softirq bio callbacks to complete,
    addressing the race condition pointed out by Chao Yu where sbi could
    be freed while f2fs_write_end_io() was still accessing sbi->cp_wait.
---
 fs/f2fs/data.c  | 11 ++++++++---
 fs/f2fs/super.c |  2 ++
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index c30e69392a62..5808d73c2598 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -318,10 +318,13 @@ static void f2fs_write_end_io(struct bio *bio)
 {
 	struct f2fs_sb_info *sbi;
 	struct folio_iter fi;
+	bool is_close;

 	iostat_update_and_unbind_ctx(bio);
 	sbi = bio->bi_private;

+	is_close = is_sbi_flag_set(sbi, SBI_IS_CLOSE);
+
 	if (time_to_inject(sbi, FAULT_WRITE_IO))
 		bio->bi_status = BLK_STS_IOERR;

@@ -360,10 +363,12 @@ static void f2fs_write_end_io(struct bio *bio)
 			f2fs_del_fsync_node_entry(sbi, folio);
 		folio_clear_f2fs_gcing(folio);
 		folio_end_writeback(folio);
-	}
-	if (!get_pages(sbi, F2FS_WB_CP_DATA) &&
+
+		if (!is_close && type == F2FS_WB_CP_DATA &&
+				!get_pages(sbi, F2FS_WB_CP_DATA) &&
 				wq_has_sleeper(&sbi->cp_wait))
-		wake_up(&sbi->cp_wait);
+			wake_up(&sbi->cp_wait);
+	}

 	bio_put(bio);
 }
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