Forwarded: Re: [syzbot] [ntfs3?] possible deadlock in ntfs_look_for_free_space

[syzbot <syzbot+d27edf9f96ae85939222@syzkaller.appspotmail.com>]

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Subject: Re: [syzbot] [ntfs3?] possible deadlock in ntfs_look_for_free_space
Author: swilczek.lx@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

From b7ce4cc69bc50a5b436e1023d682e27ea7725e16 Mon Sep 17 00:00:00 2001
From: Szymon Wilczek <swilczek.lx@gmail.com>
Date: Sat, 27 Dec 2025 04:32:44 +0100
Subject: [PATCH] ntfs3: fix circular locking dependency in run_unpack_ex

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
Closes: https://syzkaller.appspot.com/bug?extid=d27edf9f96ae85939222
Signed-off-by: Szymon Wilczek <swilczek.lx@gmail.com>
---
 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 395b20492525..dc59cad4fa37 100644
--- a/fs/ntfs3/run.c
+++ b/fs/ntfs3/run.c
@@ -1131,11 +1131,14 @@ 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)
-- 
2.52.0