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fs/ntfs3: fix deadlock in ni_read_folio_cmpr

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Syzbot reported a task hung in ni_readpage_cmpr (now ni_read_folio_cmpr).
This is caused by a lock inversion deadlock involving the inode mutex
(ni_lock) and page locks.

Scenario:
1. Task A enters ntfs_read_folio() for page X. It acquires ni_lock.
2. Task A calls ni_read_folio_cmpr(), which attempts to lock all pages in
   the compressed frame (including page Y).
3. Concurrently, Task B (e.g., via readahead) has locked page Y and
   calls ntfs_read_folio().
4. Task B waits for ni_lock (held by A).
5. Task A waits for page Y lock (held by B).
   -> DEADLOCK.

The fix is to restructure locking: do not take ni_lock in ntfs_read_folio().
Instead, acquire ni_lock inside ni_read_folio_cmpr() ONLY AFTER all required
page locks for the frame have been successfully acquired. This restores the
correct lock ordering (Page Lock -> ni_lock) consistent with VFS.

Reported-by: syzbot+5af33dd272b913b65880@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5af33dd272b913b65880
Fixes: f35590ee26 ("fs/ntfs3: remove ntfs_bio_pages and use page cache for compressed I/O")
Signed-off-by: Szymon Wilczek <swilczek.lx@gmail.com>
[almaz.alexandrovich@paragon-software.com: ni_readpage_cmpr was renamed to ni_read_folio_cmpr]
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
This commit is contained in:
Szymon Wilczek 2025-12-22 16:10:10 +01:00 committed by Konstantin Komarov
parent c613269677
commit e37a75bb86
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GPG Key ID: A9B0331F832407B6
2 changed files with 3 additions and 2 deletions
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2
fs/ntfs3/frecord.c
View File

@ -2107,7 +2107,9 @@ int ni_read_folio_cmpr(struct ntfs_inode *ni, struct folio *folio)
pages[i] = pg;
}
ni_lock(ni);
err = ni_read_frame(ni, frame_vbo, pages, pages_per_frame, 0);
ni_unlock(ni);
out1:
for (i = 0; i < pages_per_frame; i++) {

3
fs/ntfs3/inode.c
View File

@ -748,9 +748,8 @@ static int ntfs_read_folio(struct file *file, struct folio *folio)
}
if (is_compressed(ni)) {
ni_lock(ni);
/* ni_lock is taken inside ni_read_folio_cmpr after page locks */
err = ni_read_folio_cmpr(ni, folio);
ni_unlock(ni);
return err;
}