btrfs: file_remove_privs needs an exclusive lock in direct io write

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>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 6edad7b9a5d39c9a2da9b27bcdc311ceb3a3ba8f..e8726a83b6490851a2e442ecac362161d8fd4eb9 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1466,8 +1466,13 @@ 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:
@@ -1475,6 +1480,13 @@ 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(BTRFS_I(inode), ilock_flags);
+               ilock_flags &= ~BTRFS_ILOCK_SHARED;
+               goto relock;
+       }
+
        err = generic_write_checks(iocb, from);
        if (err <= 0) {
                btrfs_inode_unlock(BTRFS_I(inode), ilock_flags);