Re: FUSE: regression when clearing setuid bits on chown

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, Dec 06, 2016 at 07:13:25AM -0500, Jeff Layton wrote:

> Should we be checking that the latest i_mode even has these bits before
> sending down the mode change?

Fixed, see updated patch below.

It also fixes a bug in the previous patch where in case of "-rwsrwSr-x" it would
clear the sgid bit without execute.

> 
> > > +			attr->ia_mode = inode->i_mode & ~(S_ISUID | S_ISGID);
> > +			attr->ia_valid |= ATTR_MODE;
> >  		}
> >  	}
> >  	if (!attr->ia_valid)
> 
> Yeah that is quite a bit simpler.
> 
> That said...if either ATTR_KILL flag is set, then we're going to end up
> clearing both bits in the new mode. I guess that's ok since we always
> want to clear them both, and we'll only have one set and not the other
> if one of the mode bits was set and not the other.
> 
> But...I'm starting to wonder if we really need two flags for this. Would
> be be better served with a single ATTR_KILL_SUID_SGID flag? I wonder if
> that would simplify some of the logic in the whole setuid clearing
> morass.

Yeah, that would be a nice little cleanup.

Thanks,
Miklos
---

From: Miklos Szeredi <mszeredi@redhat.com>
Subject: fuse: fix clearing suid, sgid for chown()

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")
Cc: <stable@vger.kernel.org>
---
 fs/fuse/dir.c |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1739,8 +1739,6 @@ static int fuse_setattr(struct dentry *e
 		 * This should be done on write(), truncate() and chown().
 		 */
 		if (!fc->handle_killpriv) {
-			int kill;
-
 			/*
 			 * ia_mode calculation may have used stale i_mode.
 			 * Refresh and recalculate.
@@ -1750,12 +1748,11 @@ 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;
 			}