[PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Willem de Bruijn]

From: Willem de Bruijn <willemb@google.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@google.com>
Cc: <stable@vger.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
Fixes tag.
---
 include/linux/virtio_net.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 6c395a2600e8d..276ca543ef44d 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -173,7 +173,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 			break;
 		case SKB_GSO_TCPV4:
 		case SKB_GSO_TCPV6:
-			if (skb->csum_offset != offsetof(struct tcphdr, check))
+			if (skb->ip_summed == CHECKSUM_PARTIAL &&
+			    skb->csum_offset != offsetof(struct tcphdr, check))
 				return -EINVAL;
 			break;
 		}
-- 
2.46.0.598.g6f2099f65c-goog

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Greg KH]

On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote:
> Cc: <stable@vger.kernel.net>

This is not a correct email address :(

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Jason Wang]

On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> From: Willem de Bruijn <willemb@google.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@google.com>
> Cc: <stable@vger.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.

> Fixes tag.
> ---
>  include/linux/virtio_net.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 6c395a2600e8d..276ca543ef44d 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -173,7 +173,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>                         break;
>                 case SKB_GSO_TCPV4:
>                 case SKB_GSO_TCPV6:
> -                       if (skb->csum_offset != offsetof(struct tcphdr, check))
> +                       if (skb->ip_summed == CHECKSUM_PARTIAL &&
> +                           skb->csum_offset != offsetof(struct tcphdr, check))
>                                 return -EINVAL;
>                         break;
>                 }
> --
> 2.46.0.598.g6f2099f65c-goog
>

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Michael S. Tsirkin]

On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.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@google.com>
> Cc: <stable@vger.kernel.net>


Acked-by: Michael S. Tsirkin <mst@redhat.com>

But I think netdev maintainers ask contributors not to CC
stable directly.

> ---
> 
> 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
> Fixes tag.
> ---
>  include/linux/virtio_net.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 6c395a2600e8d..276ca543ef44d 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -173,7 +173,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>  			break;
>  		case SKB_GSO_TCPV4:
>  		case SKB_GSO_TCPV6:
> -			if (skb->csum_offset != offsetof(struct tcphdr, check))
> +			if (skb->ip_summed == CHECKSUM_PARTIAL &&
> +			    skb->csum_offset != offsetof(struct tcphdr, check))
>  				return -EINVAL;
>  			break;
>  		}
> -- 
> 2.46.0.598.g6f2099f65c-goog

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Michael S. Tsirkin]

On Tue, Sep 10, 2024 at 06:34:46AM +0200, Greg KH wrote:
> On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote:
> > Cc: <stable@vger.kernel.net>
> 
> This is not a correct email address :(

I think netdev does its own stable thing, no need to CC stable at all.

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Eric Dumazet]

On Tue, Sep 10, 2024 at 9:45 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Sep 10, 2024 at 06:34:46AM +0200, Greg KH wrote:
> > On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote:
> > > Cc: <stable@vger.kernel.net>
> >
> > This is not a correct email address :(
>
> I think netdev does its own stable thing, no need to CC stable at all.

This is no longer the case.

commit dbbe7c962c3a8163bf724dbc3c9fdfc9b16d3117
Author: Jakub Kicinski <kuba@kernel.org>
Date:   Tue Mar 2 18:46:43 2021 -0800

    docs: networking: drop special stable handling

    Leave it to Greg.

    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Willem de Bruijn]

Greg KH wrote:
> On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote:
> > Cc: <stable@vger.kernel.net>
> 
> This is not a correct email address :(
> 

Sorry. I'll resend with that fixed.

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Willem de Bruijn]

Jason Wang wrote:
> On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > From: Willem de Bruijn <willemb@google.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@google.com>
> > Cc: <stable@vger.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 == ?

> > Fixes tag.
> > ---
> >  include/linux/virtio_net.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index 6c395a2600e8d..276ca543ef44d 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -173,7 +173,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> >                         break;
> >                 case SKB_GSO_TCPV4:
> >                 case SKB_GSO_TCPV6:
> > -                       if (skb->csum_offset != offsetof(struct tcphdr, check))
> > +                       if (skb->ip_summed == CHECKSUM_PARTIAL &&
> > +                           skb->csum_offset != offsetof(struct tcphdr, check))
> >                                 return -EINVAL;
> >                         break;
> >                 }
> > --
> > 2.46.0.598.g6f2099f65c-goog
> >
> 
> Acked-by: Jason Wang <jasowang@redhat.com>

Thanks for reviewing

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Willem de Bruijn]

Willem de Bruijn wrote:
> Jason Wang wrote:
> > On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn
> > <willemdebruijn.kernel@gmail.com> wrote:
> > >
> > > From: Willem de Bruijn <willemb@google.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@google.com>
> > > Cc: <stable@vger.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.

Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

[Jason Wang]

On Tue, Sep 10, 2024 at 10:54 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> Willem de Bruijn wrote:
> > Jason Wang wrote:
> > > On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn
> > > <willemdebruijn.kernel@gmail.com> wrote:
> > > >
> > > > From: Willem de Bruijn <willemb@google.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@google.com>
> > > > Cc: <stable@vger.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.

Or it could be a physical device that can do GRO HW.

>
> 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.

Yes.

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

Thanks