Re: [Qemu-devel] [V3 PATCH 7/8] virtio-9p: Move file post creation changes to none security model

[Daniel P. Berrange]

On Thu, Jan 20, 2011 at 08:11:27PM +0530, M. Mohan Kumar wrote:
> On Thursday 20 January 2011 2:29:54 pm Stefan Hajnoczi wrote:
> > On Tue, Jan 18, 2011 at 01:54:16PM +0530, M. Mohan Kumar wrote:
> 
> > > -    if (lchown(rpath(fs_ctx, path), credp->fc_uid, credp->fc_gid) < 0) {
> > > -        /*
> > > -         * If we fail to change ownership and if we are
> > > -         * using security model none. Ignore the error
> > > -         */
> > > -        if (fs_ctx->fs_sm != SM_NONE) {
> > > -            return -1;
> > > -        }
> > > -    }
> > > +    retval = lchown(rpath(fs_ctx, path), credp->fc_uid, credp->fc_gid);
> > > 
> > >      return 0;
> > >  
> > >  }
> > 
> > retval is unused.
> > 
> 
> That was used to disable the warning message "error: ignoring return value of 
> ‘lchown’, declared with attribute warn_unused_result"
> 
> Otherwise I have to use
> if (lchown(rpath(fs_ctx, path), credp->fc_uid, credp->fc_gid)) {
>       ;
> }
> 
> > Can multiple virtio-9p requests execute at a time?  chmod() and lchown()
> > after creation is a race condition if other requests can execute
> > concurrently.
> > 
> 
> We can't implement file creation with requested user credentials and 
> permission 
> bits in the none security model atomically. Its expected behaviour only

Well you could do the nasty trick of forking a child process
and doing setuid/gid in that and then creating the file before
letting the parent continue.

  if ((pid = fork()) == 0) {
     setuid(fc_uid);
     setgid(fc_gid);
     fd =open("foo", O_CREAT);
     close(fd);
  } else {
     waitpid(pid);
  }

This kind of approach is in fact required if you want to
be able to create files with a special uid/gid on a root
squashing NFS server, because otherwise your QEMU running
as root will have its files squashed to 'nobody' when initially
created, and lchown will fail with EPERM.  You might decide
that root squashing NFS is too painful to care about supporting
though :-)

Regards,
Daniel