[PATCH 3.2 100/126] fuse: fix clearing suid, sgid for chown()

Ben Hutchings
2017-02-15T22:56:38Z

From: Miklos Szeredi <mszeredi@redhat.com>

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@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: a09f99eddef4 ("fuse: fix killing s[ug]id in setattr")
Reviewed-by: Jeff Layton <jlayton@redhat.com>
[bwh: Backported to 3.2: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/fuse/dir.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1398,8 +1398,6 @@ static int fuse_setattr(struct dentry *e
int ret;

if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) {
- int kill;
-
attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID |
ATTR_MODE);
/*
@@ -1411,12 +1409,11 @@ 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;
}