Subject: Re: [PATCH net] xfrm: Clear sk_dst_cache when applying per-socket policy.

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 16 Aug 2017 13:19:53 +0200

On Wed, 16 Aug 2017 03:43:54 -0700
Eric Dumazet <[email-masked]> wrote:

> On Wed, 2017-08-16 at 11:03 +0200, Jakub Sitnicki wrote:
> > On Tue, 15 Aug 2017 15:25:10 -0700
> > Jonathan Basseri <[email-masked]> wrote:
> >   
> > > If an IPv6 socket has a valid dst cache, then xfrm_lookup_route will get
> > > skipped. However, the cache is not invalidated when applying policy to a
> > > socket (i.e. IPV6_XFRM_POLICY). The result is that new policies are
> > > sometimes ignored on those sockets.
> > > 
> > > This can be demonstrated like so,
> > > 1. Create UDPv6 socket.
> > > 2. connect() the socket.
> > > 3. Apply an outbound XFRM policy to the socket.
> > > 4. send() data on the socket.
> > > 
> > > Packets will continue to be sent in the clear instead of matching an
> > > xfrm or returning a no-match error (EAGAIN). This affects calls to
> > > send() and not sendto().
> > > 
> > > Note: Creating normal XFRM policies should have a similar effect on
> > > sk_dst_cache entries that match the policy, but that is not fixed in
> > > this patch.
> > > 
> > > Fixes: 00bc0ef5880d ("ipv6: Skip XFRM lookup if dst_entry in socket cache is valid")
> > > Tested: https://android-review.googlesource.com/418659
> > > Signed-off-by: Jonathan Basseri <[email-masked]>
> > > ---  
> > 
> > Thank you for the fix.
> > 
> > Acked-by: Jakub Sitnicki <[email-masked]>  
> 
> I do not believe this fix is correct.
> 
> What happens if the socket is TCP ?
> 
> sk_dst_reset(sk) is not safe for them.
> 
> This might add use-after-free, and eventually crash.

You are right. I see that RCU-variant __sk_dst_reset() is used
throughout TCP stack. Thank you for pointing it out.

Please disregard my earlier ACK.

-Jakub