btrfs: file_remove_privs needs an exclusive lock in direct io write

commit 9af86694fd5d387992699ec99007ed374966ce9a upstream.

This was noticed by Miklos that file_remove_privs might call into
notify_change(), which requires to hold an exclusive lock. The problem
exists in FUSE and btrfs. We can fix it without any additional helpers
from VFS, in case the privileges would need to be dropped, change the
lock type to be exclusive and redo the loop.

Fixes: e9adabb9712e ("btrfs: use shared lock for direct writes within EOF")
CC: Miklos Szeredi <miklos@szeredi.hu>
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 77202addead83834925e0b8705df1f49020c5f99..0a46fff3dd0679d07d00bfecf8e3c94a5aa411cf 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1458,8 +1458,13 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
        if (iocb->ki_flags & IOCB_NOWAIT)
                ilock_flags |= BTRFS_ILOCK_TRY;
 
-       /* If the write DIO is within EOF, use a shared lock */
-       if (iocb->ki_pos + iov_iter_count(from) <= i_size_read(inode))
+       /*
+        * If the write DIO is within EOF, use a shared lock and also only if
+        * security bits will likely not be dropped by file_remove_privs() called
+        * from btrfs_write_check(). Either will need to be rechecked after the
+        * lock was acquired.
+        */
+       if (iocb->ki_pos + iov_iter_count(from) <= i_size_read(inode) && IS_NOSEC(inode))
                ilock_flags |= BTRFS_ILOCK_SHARED;
 
 relock:
@@ -1467,6 +1472,13 @@ relock:
        if (err < 0)
                return err;
 
+       /* Shared lock cannot be used with security bits set. */
+       if ((ilock_flags & BTRFS_ILOCK_SHARED) && !IS_NOSEC(inode)) {
+               btrfs_inode_unlock(inode, ilock_flags);
+               ilock_flags &= ~BTRFS_ILOCK_SHARED;
+               goto relock;
+       }
+
        err = generic_write_checks(iocb, from);
        if (err <= 0) {
                btrfs_inode_unlock(inode, ilock_flags);