netdev - Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <66e05d81c04fe_a00b829435@willemb.c.googlers.com.notmuch>
Date: Tue, 10 Sep 2024 10:53:53 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Jason Wang <jasowang@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, 
 davem@...emloft.net, 
 kuba@...nel.org, 
 edumazet@...gle.com, 
 pabeni@...hat.com, 
 stable@...r.kernel.org, 
 nsz@...t70.net, 
 mst@...hat.com, 
 yury.khrustalev@....com, 
 broonie@...nel.org, 
 sudeep.holla@....com, 
 Willem de Bruijn <willemb@...gle.com>, 
 stable@...r.kernel.net
Subject: Re: [PATCH net] net: tighten bad gso csum offset check in
 virtio_net_hdr

Willem de Bruijn wrote:
> Jason Wang wrote:
> > On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > From: Willem de Bruijn <willemb@...gle.com>
> > >
> > > The referenced commit drops bad input, but has false positives.
> > > Tighten the check to avoid these.
> > >
> > > The check detects illegal checksum offload requests, which produce
> > > csum_start/csum_off beyond end of packet after segmentation.
> > >
> > > But it is based on two incorrect assumptions:
> > >
> > > 1. virtio_net_hdr_to_skb with VIRTIO_NET_HDR_GSO_TCP[46] implies GSO.
> > > True in callers that inject into the tx path, such as tap.
> > > But false in callers that inject into rx, like virtio-net.
> > > Here, the flags indicate GRO, and CHECKSUM_UNNECESSARY or
> > > CHECKSUM_NONE without VIRTIO_NET_HDR_F_NEEDS_CSUM is normal.
> > >
> > > 2. TSO requires checksum offload, i.e., ip_summed == CHECKSUM_PARTIAL.
> > > False, as tcp[46]_gso_segment will fix up csum_start and offset for
> > > all other ip_summed by calling __tcp_v4_send_check.
> > >
> > > Because of 2, we can limit the scope of the fix to virtio_net_hdr
> > > that do try to set these fields, with a bogus value.
> > >
> > > Link: https://lore.kernel.org/netdev/20240909094527.GA3048202@port70.net/
> > > Fixes: 89add40066f9 ("net: drop bad gso csum_start and offset in virtio_net_hdr")
> > > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > > Cc: <stable@...r.kernel.net>
> > >
> > > ---
> > >
> > > Verified that the syzbot repro is still caught.
> > >
> > > An equivalent alternative would be to move the check for csum_offset
> > > to where the csum_start check is in segmentation:
> > >
> > > -    if (unlikely(skb_checksum_start(skb) != skb_transport_header(skb)))
> > > +    if (unlikely(skb_checksum_start(skb) != skb_transport_header(skb) ||
> > > +                 skb->csum_offset != offsetof(struct tcphdr, check)))
> > >
> > > Cleaner, but messier stable backport.
> > >
> > > We'll need an equivalent patch to this for VIRTIO_NET_HDR_GSO_UDP_L4.
> > > But that csum_offset test was in a different commit, so different
> > 
> > Not for this patch, but I see this in UDP_L4:
> > 
> >                        if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
> >                                return -EINVAL;
> > 
> > This seems to forbid VIRTIO_NET_HDR_F_DATA_VALID. I wonder what's the
> > reason for doing this.
> 
> It tests &, not == ?

Oh you mean as alternative, for receive of GRO from hypervisor.

Yes, fair point.

Then we also trust a privileged process over tun, like syzkaller.
When it comes to checksums, I suppose that is fine: it cannot harm
kernel integrity.

One missing piece is that TCP GSO will fix up non CHECKSUM_PARTIAL
skbs. UDP GSO does not have the same logic.