[PATCH] fs/ntfs3: disable page fault during ntfs_fiemap()

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

syzbot is reporting circular locking dependency between ntfs_file_mmap()
(which has mm->mmap_lock => ni->ni_lock dependency) and ntfs_fiemap()
(which has ni->ni_lock => mm->mmap_lock dependency).

Since ni_fiemap() is called by ioctl(FS_IOC_FIEMAP) via optional
"struct inode_operations"->fiemap callback, I assume that importance of
ni_fiemap() is lower than ntfs_file_mmap().

Also, since Documentation/filesystems/fiemap.rst says that "If an error
is encountered while copying the extent to user memory, -EFAULT will be
returned.", I assume that ioctl(FS_IOC_FIEMAP) users can handle -EFAULT
error.

Therefore, in order to eliminate possibility of deadlock, until

  Assumed ni_lock.
  TODO: Less aggressive locks.

comment in ni_fiemap() is removed, use ni_fiemap() with best-effort basis
(i.e. fail with -EFAULT when a page fault is inevitable).

Reported-by: syzbot <syzbot+96cee7d33ca3f87eee86@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=96cee7d33ca3f87eee86
Fixes: 4342306f0f0d ("fs/ntfs3: Add file operations and implementation")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 fs/ntfs3/file.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/ntfs3/file.c b/fs/ntfs3/file.c
index e9bdc1ff08c9..a9e7204e1579 100644
--- a/fs/ntfs3/file.c
+++ b/fs/ntfs3/file.c
@@ -1146,9 +1146,11 @@ int ntfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 		return err;
 
 	ni_lock(ni);
+	pagefault_disable();
 
 	err = ni_fiemap(ni, fieinfo, start, len);
 
+	pagefault_enable();
 	ni_unlock(ni);
 
 	return err;
-- 
2.34.1