[Virtio-fs] [PATCH v2 4/6] fuse: Kill suid/sgid using ATTR_MODE if it is not truncate

[Vivek Goyal]

On Tue, Sep 22, 2020 at 11:25:30PM +0200, Miklos Szeredi wrote:
> On Tue, Sep 22, 2020 at 10:08 PM Vivek Goyal <vgoyal at redhat.com> wrote:
> >
> > On Tue, Sep 22, 2020 at 03:56:47PM +0200, Miklos Szeredi wrote:
> > > On Wed, Sep 16, 2020 at 6:18 PM Vivek Goyal <vgoyal at redhat.com> wrote:
> > >
> > > > But if this is non-truncate setattr then server will not kill suid/sgid.
> > > > So continue to send ATTR_MODE to kill suid/sgid for non-truncate setattr,
> > > > even if ->handle_killpriv_v2 is enabled.
> > >
> > > Sending ATTR_MODE doesn't make sense, since that is racy.   The
> > > refresh-recalculate makes the race window narrower, but it doesn't
> > > eliminate it.
> >
> > Hi Miklos,
> >
> > Agreed that it does not eliminate that race.
> >
> > >
> > > I think I suggested sending write synchronously if suid/sgid/caps are
> > > set.  Do you see a problem with this?
> >
> > Sorry, I might have missed it. So you are saying that for the case of
> > ->writeback_cache, force a synchronous WRITE if suid/sgid is set. But
> > this will only work if client sees the suid/sgid bits. If client B
> > set the suid/sgid which client A does not see then all the WRITEs
> > will be cached in client A and not clear suid/sgid bits.
> 
> Unless the attributes are invalidated (either by timeout or
> explicitly) there's no way that in that situation the suid/sgid bits
> can be cleared.  That's true of your patch as well.

Right. And that's why I mentioned that handle_killpriv_v2 is not fully
compatible with ->writeback_cache.

> 
> >
> > Also another problem is that if client sees suid/sgid and we make
> > WRITE synchronous, client's suid/sgid attrs are still cached till
> > next refresh (both for ->writeback_cache and non writeback_cache
> > case). So server is clearing suid/sgid bits but client still
> > keeps them cached. I hope none of the code paths end up using
> > this stale value and refresh attrs before using suid/sgid.
> >
> > Shall we refresh attrs after WRITE if suid/sgid is set and client
> > expects it to clear after WRITE finishes to solve this problem. Or
> > this is something which is actually not a real problem and I am
> > overdesigning.
> 
> The fuse_perform_write() path already has the attribute invalidation,
> which will trigger GETATTR from fuse_update_attributes() in the next
> write.

Ok. So if there is any path which potentially can make use of cached
suid/sgid, we just need to make sure fuse_update_attributes() has been
called in that path.

> 
> So I think all that should work fine.

Sounds good. I will give it a try and see if I notice any other issues.

Thanks
Vivek