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>

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index f3e4fbb59985573b80168d0b00d9aa2f22993faa..71c2c712e609afca2e9f5c93049ca2e641221108 100644 (file)
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -563,6 +563,13 @@ restart:
        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 @@ restart:
         * 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;
 }
 
 /*