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; }
Hi,
Thanks for your patch.
FYI: kernel test robot notices the stable kernel rule is not satisfied.
The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#opti...
Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree. Subject: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr Link: https://lore.kernel.org/stable/20240910004033.530313-1-willemdebruijn.kernel...
On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote:
This is not a correct email address :(
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:
This is not a correct email address :(
I think netdev does its own stable thing, no need to CC stable at all.
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:
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
Greg KH wrote:
On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote:
This is not a correct email address :(
Sorry. I'll resend with that fixed.
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:
- 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.
- 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
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:
- 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.
- 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
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:
- 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.
- 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.
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:
- 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.
- 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
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:
- 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.
- 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
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