knfsd: only set ATTR_KILL_S*ID if ATTR_MODE isn't being explicitly set

It's theoretically possible for a single SETATTR call to come in that sets the
mode and the uid/gid.  In that case, don't set the ATTR_KILL_S*ID bits since
that would trip the BUG() in notify_change.  Just fix up the mode to have the
same effect.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Neil Brown <neilb@suse.de>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfsd/vfs.c

@@ -364,14 +364,23 @@ nfsd_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp, struct iattr *iap,
 	if (iap->ia_valid & ATTR_MODE) {
 		iap->ia_mode &= S_IALLUGO;
 		imode = iap->ia_mode |= (imode & ~S_IALLUGO);
+		/* if changing uid/gid revoke setuid/setgid in mode */
+		if ((iap->ia_valid & ATTR_UID) && iap->ia_uid != inode->i_uid) {
+			iap->ia_valid |= ATTR_KILL_PRIV;
+			iap->ia_mode &= ~S_ISUID;
+		}
+		if ((iap->ia_valid & ATTR_GID) && iap->ia_gid != inode->i_gid)
+			iap->ia_mode &= ~S_ISGID;
+	} else {
+		/*
+		 * Revoke setuid/setgid bit on chown/chgrp
+		 */
+		if ((iap->ia_valid & ATTR_UID) && iap->ia_uid != inode->i_uid)
+			iap->ia_valid |= ATTR_KILL_SUID | ATTR_KILL_PRIV;
+		if ((iap->ia_valid & ATTR_GID) && iap->ia_gid != inode->i_gid)
+			iap->ia_valid |= ATTR_KILL_SGID;
 	}
 
-	/* Revoke setuid/setgid bit on chown/chgrp */
-	if ((iap->ia_valid & ATTR_UID) && iap->ia_uid != inode->i_uid)
-		iap->ia_valid |= ATTR_KILL_SUID | ATTR_KILL_PRIV;
-	if ((iap->ia_valid & ATTR_GID) && iap->ia_gid != inode->i_gid)
-		iap->ia_valid |= ATTR_KILL_SGID;
-
 	/* Change the attributes. */
 
 	iap->ia_valid |= ATTR_CTIME;