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ntfs3: fix circular locking dependency in run_unpack_ex

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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>
This commit is contained in:
Szymon Wilczek 2025-12-27 15:43:07 +01:00 committed by Konstantin Komarov
parent 099ef9ab92
commit 08ce2fee1b
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GPG Key ID: A9B0331F832407B6
1 changed files with 8 additions and 5 deletions
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13
fs/ntfs3/run.c
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@ -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)