[PATCH 3.16 280/306] fuse: fix clearing suid, sgid for chown()

[Ben Hutchings]

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Miklos Szeredi <mszeredi@xxxxxxxxxx>

commit c01638f5d919728f565bf8b5e0a6a159642df0d9 upstream.

Basically, the pjdfstests set the ownership of a file to 06555, and then
chowns it (as root) to a new uid/gid. Prior to commit a09f99eddef4 ("fuse:
fix killing s[ug]id in setattr"), fuse would send down a setattr with both
the uid/gid change and a new mode.  Now, it just sends down the uid/gid
change.

Technically this is NOTABUG, since POSIX doesn't _require_ that we clear
these bits for a privileged process, but Linux (wisely) has done that and I
think we don't want to change that behavior here.

This is caused by the use of should_remove_suid(), which will always return
0 when the process has CAP_FSETID.

In fact we really don't need to be calling should_remove_suid() at all,
since we've already been indicated that we should remove the suid, we just
don't want to use a (very) stale mode for that.

This patch should fix the above as well as simplify the logic.

Reported-by: Jeff Layton <jlayton@xxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Fixes: a09f99eddef4 ("fuse: fix killing s[ug]id in setattr")
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
[bwh: Backported to 3.16: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/fuse/dir.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1829,8 +1829,6 @@ static int fuse_setattr(struct dentry *e
 		return -EACCES;
 
 	if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) {
-		int kill;
-
 		attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID |
 				    ATTR_MODE);
 		/*
@@ -1842,12 +1840,11 @@ static int fuse_setattr(struct dentry *e
 			return ret;
 
 		attr->ia_mode = inode->i_mode;
-		kill = should_remove_suid(entry);
-		if (kill & ATTR_KILL_SUID) {
+		if (inode->i_mode & S_ISUID) {
 			attr->ia_valid |= ATTR_MODE;
 			attr->ia_mode &= ~S_ISUID;
 		}
-		if (kill & ATTR_KILL_SGID) {
+		if ((inode->i_mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
 			attr->ia_valid |= ATTR_MODE;
 			attr->ia_mode &= ~S_ISGID;
 		}