xfs: Correctly lock inode when removing suid and file capabilities

Currently XFS calls file_remove_privs() without holding i_mutex. This is
wrong because that function can end up messing with file permissions and
file capabilities stored in 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.
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/xfs/xfs_file.c

@@ -563,6 +563,13 @@ xfs_file_aio_write_checks(
 	if (error)
 		return error;
 
+	/* For changing security info in file_remove_privs() we need i_mutex */
+	if (*iolock == XFS_IOLOCK_SHARED && !IS_NOSEC(inode)) {
+		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
@@ -623,7 +630,9 @@ xfs_file_aio_write_checks(
 	 * setgid bits if the process is not being run by root.  This keeps
 	 * people from modifying setuid and setgid binaries.
 	 */
-	return file_remove_privs(file);
+	if (!IS_NOSEC(inode))
+		return file_remove_privs(file);
+	return 0;
 }
 
 /*