[PATCH] xfs: Correctly lock inode when removing suid and security marks

[Jan Kara]

Currently XFS calls file_remove_suid() without holding i_mutex. This is
wrong because that function can end up messing with file permissions and
security xattrs for which we need i_mutex held.

Fix the problem by grabbing iolock exclusively when we will need to
change anything in permissions / xattrs.

Signed-off-by: Jan Kara <jack at suse.cz>
---
 fs/xfs/xfs_file.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index eb596b419942..ad6636ac4943 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -521,6 +521,18 @@ restart:
 	if (error)
 		return error;
 
+	/* For changing security info in file_remove_suid() we need i_mutex */
+	if (!IS_NOSEC(inode) && *iolock == XFS_IOLOCK_SHARED) {
+		struct dentry *dentry = file->f_path.dentry;
+
+		if (should_remove_suid(dentry) ||
+		    security_inode_need_killpriv(dentry)) {
+			xfs_rw_iunlock(ip, *iolock);
+			*iolock = XFS_IOLOCK_EXCL;
+			xfs_rw_ilock(ip, *iolock);
+			goto restart;
+		}
+	}
 	/*
 	 * If the offset is beyond the size of the file, we need to zero any
 	 * blocks that fall between the existing EOF and the start of this
-- 
1.8.1.4