Re: [PATCH 7/8] conf, passt, tap: Open socket and PID files before switching UID/GID

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 20 Jun 2024 13:47:31 +0100
"Richard W.M. Jones" <rjones@redhat.com> wrote:

> On Thu, Jun 20, 2024 at 02:12:53PM +0200, Stefano Brivio wrote:
> > On Thu, 20 Jun 2024 12:30:54 +0100
> > "Richard W.M. Jones" <rjones@redhat.com> wrote:
> >   
> > > On Wed, May 29, 2024 at 12:35:24PM +1000, David Gibson wrote:  
> > > > On Wed, May 22, 2024 at 10:59:10PM +0200, Stefano Brivio wrote:    
> > > > > Otherwise, if the user runs us as root, and gives us paths that are
> > > > > only accessible by root, we'll fail to open them, which might in turn
> > > > > encourage users to change permissions or ownerships: definitely a bad
> > > > > idea in terms of security.
> > > > > 
> > > > > Reported-by: Minxi Hou <mhou@redhat.com>
> > > > > Reported-by: Richard W.M. Jones <rjones@redhat.com>
> > > > > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>    
> > > > 
> > > > Looking at this I did notice a pre-existing, well, maybe not bug
> > > > exactly, but possibly surprising behaviour, which makes me a
> > > > bit more nervous now that we can invoke it as root.
> > > > 
> > > > tap_sock_unix_open() will silently truncate the given socket path to
> > > > the maximum length for a Unix socket.  Which means we could bind(),
> > > > but also unlink() a path that's not exactly the same as the one the
> > > > one the user requested.  I don't immediately see a way to exploit
> > > > that, but it's the sort of thing that makes me nervous.  I think we
> > > > should instead outright fail if the given socket path is too long.    
> > > 
> > > Yes, agreed.
> > > 
> > > It seems as if the latest passt code still does this.  Do you want me
> > > to open a bug about it?  
> > 
> > Yes, please, that, or a patch :)  
> 
> While I was testing this, I found we do seem to check it:
> 
> https://passt.top/passt/tree/conf.c#n1446

Oh, I thought David was referring to the loop in tap_sock_unix_open(),
where we try paths in the form "/tmp/passt_%i.socket". But even there,
we can't exceed UNIX_PATH_MAX.

One minor issue remains, though: in conf(), we refuse paths that are
longer than UNIX_SOCK_MAX (100). That's the maximum index for the
"/tmp/passt_%i.socket", it happens to be a sane value, but we should use
UNIX_PATH_MAX (108) instead. I'll fix it, but wait for David's feedback
first.

-- 
Stefano