netdev - Re: [PATCH net-next] udp_tunnel: properly deal with xfrm gro encap.

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Message-ID: <67dc1d3d60383_a04b129455@willemb.c.googlers.com.notmuch>
Date: Thu, 20 Mar 2025 09:50:53 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Paolo Abeni <pabeni@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>, 
 David Ahern <dsahern@...nel.org>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Simon Horman <horms@...nel.org>, 
 Willem de Bruijn <willemb@...gle.com>, 
 steffen.klassert@...unet.com
Subject: Re: [PATCH net-next] udp_tunnel: properly deal with xfrm gro encap.

Paolo Abeni wrote:
> On 3/19/25 6:44 PM, Willem de Bruijn wrote:
> > Paolo Abeni wrote:
> >> Given syzkaller has found another splat with no reproducer on the other
> >> UDP GRO change of mine [1] and we are almost at merge window time, I'm
> >> considering reverting entirely such changes and re-submit later
> >> (hopefully fixed). WDYT?
> > 
> > Your call. I suspect that we can forward fix this. But yes, that is
> > always the riskier approach. And from a first quick look at the
> > report, the right fix is not immediately glaringly obvious indeed.
> 
> One problem to me is that I have my hands significantly full, since the
> revert looks like the faster way out it looks the more appealing
> candidate to me.
> 
> WRT the other issue, I think the problem is in udp_tunnel_cleanup_gro();
> this check:
> 
>         if (!up->tunnel_list.pprev)
>                 return;
> 
> at sk deletion time is performed outside any lock. The current CPU could
> see the old list value (empty) even if another core previously added the
> sk into the UDP tunnel GRO list, thus skipping the removal from such
> list. Later list operation will do UaF while touching the same list.
> 
> Moving the check under the udp_tunnel_gro_lock spinlock should solve the
> issue.

FWIW, the full revert doesn't have to happen in the net-next timeframe.
Can always revert to that in -rcX. And try a forward fix first, after
the merge is over.