linux-kernel - [PATCH] fs/ntfs3: don't hold ni_lock when calling truncate

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Date:   Mon, 2 Jan 2023 23:05:33 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Konstantin Komarov <almaz.alexandrovich@...agon-software.com>,
        ntfs3@...ts.linux.dev
Cc:     syzbot <syzbot+bed15dbf10294aa4f2ae@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com, Hillf Danton <hdanton@...a.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] fs/ntfs3: don't hold ni_lock when calling truncate_setsize()

syzbot is reporting hung task at do_user_addr_fault() [1], for
there is a silent deadlock between PG_locked bit and ni_lock lock.

Since filemap_update_page() calls filemap_read_folio() after calling
folio_trylock() which will set PG_locked bit, ntfs_truncate() must not
call truncate_setsize() which will wait for PG_locked bit to be
cleared when holding ni_lock lock.

Link: https://syzkaller.appspot.com/bug?extid=bed15dbf10294aa4f2ae [1]
Reported-by: syzbot <syzbot+bed15dbf10294aa4f2ae@...kaller.appspotmail.com>
Debugged-by: Linus Torvalds <torvalds@...ux-foundation.org>
Co-developed-by: Hillf Danton <hdanton@...a.com>
Signed-off-by: Hillf Danton <hdanton@...a.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Fixes: 4342306f0f0d ("fs/ntfs3: Add file operations and implementation")
---
By the way, ntfs_truncate() is ignoring attr_set_size() failure. Is it OK?

 fs/ntfs3/file.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ntfs3/file.c b/fs/ntfs3/file.c
index e5399ebc3a2b..d294cd975688 100644
--- a/fs/ntfs3/file.c
+++ b/fs/ntfs3/file.c
@@ -390,10 +390,10 @@ static int ntfs_truncate(struct inode *inode, loff_t new_size)
 
 	new_valid = ntfs_up_block(sb, min_t(u64, ni->i_valid, new_size));
 
-	ni_lock(ni);
-
 	truncate_setsize(inode, new_size);
 
+	ni_lock(ni);
+
 	down_write(&ni->file.run_lock);
 	err = attr_set_size(ni, ATTR_DATA, NULL, 0, &ni->file.run, new_size,
 			    &new_valid, ni->mi.sbi->options->prealloc, NULL);
-- 
2.34.1