Re: [PATCH net] gso: fix GSO_DODGY bit handling for related protocols

[Yan Zhai <yan@cloudflare.com>]

On Wed, Jul 12, 2023 at 9:02 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> On Wed, Jul 12, 2023 at 9:55 PM Yan Zhai <yan@cloudflare.com> wrote:
> >
> > SKB_GSO_DODGY bit indicates a GSO packet comes from an untrusted source.
> > The canonical way is to recompute the gso_segs to avoid device driver
> > issues. Afterwards, the DODGY bit can be removed to avoid re-check at the
> > egress of later devices, e.g. packets can egress to a vlan device backed
> > by a real NIC.
> >
> > Commit 1fd54773c267 ("udp: allow header check for dodgy GSO_UDP_L4
> > packets.") checks DODGY bit for UDP, but for packets that can be fed
> > directly to the device after gso_segs reset, it actually falls through
> > to fragmentation [1].
> >
> > Commit 90017accff61 ("sctp: Add GSO support") and commit 3820c3f3e417
> > ("[TCP]: Reset gso_segs if packet is dodgy") both didn't remove the DODGY
> > bit after recomputing gso_segs.
> >
> > This change fixes the GSO_UDP_L4 handling case, and remove the DODGY bit
> > at other places.
>
> These two things should not be conflated.
>
> Only the USO fix is strictly needed to fix the reported issue.
>
It's my OCD of wanting to avoid a cover letter for two patches...
Let's address just this UDP issue then this time. The removal of DODGY
is in fact more suitable as RFC for small improvements.

> > Fixes: 90017accff61 ("sctp: Add GSO support")
> > Fixes: 3820c3f3e417 ("[TCP]: Reset gso_segs if packet is dodgy")
> > Fixes: 1fd54773c267 ("udp: allow header check for dodgy GSO_UDP_L4 packets.")
>
> Link: https://lore.kernel.org/all/CAJPywTKDdjtwkLVUW6LRA2FU912qcDmQOQGt2WaDo28KzYDg+A@mail.gmail.com/
>
> > Signed-off-by: Yan Zhai <yan@cloudflare.com>
> >
> > ---
> > [1]:
> > https://lore.kernel.org/all/CAJPywTKDdjtwkLVUW6LRA2FU912qcDmQOQGt2WaDo28KzYDg+A@mail.gmail.com/
> >
> > ---
> >  net/ipv4/tcp_offload.c |  1 +
> >  net/ipv4/udp_offload.c | 19 +++++++++++++++----
> >  net/ipv6/udp_offload.c | 19 +++++++++++++++----
> >  net/sctp/offload.c     |  2 ++
> >  4 files changed, 33 insertions(+), 8 deletions(-)
> >
> > diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> > index 8311c38267b5..f9b93708c22e 100644
> > --- a/net/ipv4/tcp_offload.c
> > +++ b/net/ipv4/tcp_offload.c
> > @@ -87,6 +87,7 @@ struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
> >                 /* Packet is from an untrusted source, reset gso_segs. */
> >
> >                 skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);
> > +               skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
> >
> >                 segs = NULL;
> >                 goto out;
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index 75aa4de5b731..bd29cf19bb6b 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -388,11 +388,22 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
> >         if (!pskb_may_pull(skb, sizeof(struct udphdr)))
> >                 goto out;
> >
> > -       if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> > -           !skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST))
> > -               return __udp_gso_segment(skb, features, false);
> > -
> >         mss = skb_shinfo(skb)->gso_size;
>
> Why move the block below this line?
>
if we move the dodgy handling into __udp_gso_segment then it does not
need to move below this line.

> > +
> > +       if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
> > +               if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
> > +                       /* Packet is from an untrusted source, reset actual gso_segs */
> > +                       skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len - sizeof(*uh),
> > +                                                                mss);
> > +                       skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
> > +
> > +                       segs = NULL;
> > +                       goto out;
> > +               } else {
> > +                       return __udp_gso_segment(skb, features, false);
> > +               }
> > +       }
> > +
>
> The validation should take place inside __udp_gso_segment.
>
> Revert the previous patch to always enter that function for USO packets:
>
>        if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4)
>                 return __udp_gso_segment(skb, features, false);
>
> And in that function decide to return NULL after validation.
>
Good call, that's indeed better. Thanks

>
> >         if (unlikely(skb->len <= mss))
> >                 goto out;
> >
> > diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
> > index ad3b8726873e..6857d9f7bd06 100644
> > --- a/net/ipv6/udp_offload.c
> > +++ b/net/ipv6/udp_offload.c
> > @@ -43,11 +43,22 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
> >                 if (!pskb_may_pull(skb, sizeof(struct udphdr)))
> >                         goto out;
> >
> > -               if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> > -                   !skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST))
> > -                       return __udp_gso_segment(skb, features, true);
> > -
> >                 mss = skb_shinfo(skb)->gso_size;
> > +
> > +               if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
> > +                       if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
> > +                               /* Packet is from an untrusted source, reset actual gso_segs */
> > +                               skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len - sizeof(*uh),
> > +                                                                        mss);
> > +                               skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
> > +
> > +                               segs = NULL;
> > +                               goto out;
> > +                       } else {
> > +                               return __udp_gso_segment(skb, features, true);
> > +                       }
> > +               }
> > +
> >                 if (unlikely(skb->len <= mss))
> >                         goto out;
> >
> > diff --git a/net/sctp/offload.c b/net/sctp/offload.c
> > index 502095173d88..3d2b44db0d42 100644
> > --- a/net/sctp/offload.c
> > +++ b/net/sctp/offload.c
> > @@ -65,6 +65,8 @@ static struct sk_buff *sctp_gso_segment(struct sk_buff *skb,
> >                 skb_walk_frags(skb, frag_iter)
> >                         pinfo->gso_segs++;
> >
> > +               pinfo->gso_type &= ~SKB_GSO_DODGY;
> > +
> >                 segs = NULL;
> >                 goto out;
> >         }
> > --
> > 2.30.2
> >



-- 

Yan