[PATCH 6.18 362/752] ntfs3: fix circular locking dependency in run_unpack_ex

Sasha Levin
2026-02-28T17:53:11Z

From: Szymon Wilczek <swilczek.lx@gmail.com>

[ Upstream commit 08ce2fee1b869ecbfbd94e0eb2630e52203a2e03 ]

Syzbot reported a circular locking dependency between wnd->rw_lock
(sbi->used.bitmap) and ni->file.run_lock.

The deadlock scenario:
1. ntfs_extend_mft() takes ni->file.run_lock then wnd->rw_lock.
2. run_unpack_ex() takes wnd->rw_lock then tries to acquire
ni->file.run_lock inside ntfs_refresh_zone().

This creates an AB-BA deadlock.

Fix this by using down_read_trylock() instead of down_read() when
acquiring run_lock in run_unpack_ex(). If the lock is contended,
skip ntfs_refresh_zone() - the MFT zone will be refreshed on the
next MFT operation. This breaks the circular dependency since we
never block waiting for run_lock while holding wnd->rw_lock.

Reported-by: syzbot+d27edf9f96ae85939222@syzkaller.appspotmail.com
Tested-by: syzbot+d27edf9f96ae85939222@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d27edf9f96ae85939222
Signed-off-by: Szymon Wilczek <swilczek.lx@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ntfs3/run.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/ntfs3/run.c b/fs/ntfs3/run.c
index 5df55e4adbb11..21decc2922823 100644
--- a/fs/ntfs3/run.c
+++ b/fs/ntfs3/run.c
@@ -1124,11 +1124,14 @@ int run_unpack_ex(struct runs_tree *run, struct ntfs_sb_info *sbi, CLST ino,
struct rw_semaphore *lock =
is_mounted(sbi) ? &sbi->mft.ni->file.run_lock :
NULL;
- if (lock)
- down_read(lock);
- ntfs_refresh_zone(sbi);
- if (lock)
- up_read(lock);
+ if (lock) {
+ if (down_read_trylock(lock)) {
+ ntfs_refresh_zone(sbi);
+ up_read(lock);
+ }
+ } else {
+ ntfs_refresh_zone(sbi);
+ }
}
up_write(&wnd->rw_lock);
if (err)
--
2.51.0