From 9af86694fd5d387992699ec99007ed374966ce9a Mon Sep 17 00:00:00 2001 From: Bernd Schubert Date: Wed, 6 Sep 2023 17:59:03 +0200 Subject: 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 CC: stable@vger.kernel.org # 5.15+ Reviewed-by: Christoph Hellwig Signed-off-by: Bernd Schubert Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/file.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) (limited to 'fs/btrfs/file.c') diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index 6edad7b9a5d3..e8726a83b649 100644 --- 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); -- cgit