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Fri, 16 Oct 2026 02:34:53 -0700 (PDT) Date: Fri, 16 Oct 2026 05:34:50 -0400 From: "Michael S. Tsirkin" To: Chris Arges Cc: Willem de Bruijn , linux-kernel@vger.kernel.org, Jason Wang , Eugenio =?iso-8859-1?Q?P=E9rez?= , Xuan Zhuo , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, virtualization@lists.linux.dev, Rusty Russell , Max Krasnyansky , Paulos Yibelo Subject: Re: [PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress Message-ID: <20261016053324-mutt-send-email-mst@kernel.org> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Oct 15, 2026 at 02:18:24PM -0500, Chris Arges wrote: > On 2026-10-15 13:43:56, Willem de Bruijn wrote: > > Michael S. Tsirkin wrote: > > > As reported first by Paulos Yibelo (after which > > > others, credited below, found more ways the bug can affect users) a > > > virtio net header can place CHECKSUM_PARTIAL at any in-packet offset. > > > If that state is retained, later checksum completion can modify the > > > packet after protocol processing has made decisions from it. > > > > > > Let's fix that: let's validate csum_start and csum_offset and only keep > > > CHECKSUM_PARTIAL when csum_start and csum_offset match a parsed TCP or > > > UDP header, while handling VLAN headers, IPv4 options and IPv6 extension > > > headers. Complete other non-GSO checksums immediately, before any > > > protocol processing runs. > > > > > > We can't easily "complete the checksum" for GSO we don't recognize - GSO > > > assumes knowledge of the protocol. We could segment ignoring the > > > checksum then do whatever csum_start and csum_offset say, but that > > > seems like an overkill for something that shouldn't happen. > > > > > > There's some risk here: > > > - non-gso packets which can't be parsed get checksum calculated in > > > software, adding performance overhead > > > - gso packets which can't be parsed get dropped > > > > > > To mitigate the risks, the following patch will add a sysctl knob to > > > bypass the resolution. This is reasonable if the setup is such that > > > there is no protocol processing at all. > > > > > > UDP tunnel headers get the same handling on the inner header. > > > > > > Packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are unaffected. > > > > > > Assisted-by: LLM > > > Reported-by: Paulos Yibelo > > > Reported-by: Chris Arges > > > Tested-by: Chris Arges > > > Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr") > > > Suggested-by: Willem de Bruijn > > > Signed-off-by: Michael S. Tsirkin > > > --- > > > include/linux/virtio_net.h | 251 +++++++++++++++++++++++++++++++++++-- > > > 1 file changed, 241 insertions(+), 10 deletions(-) > > > > > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h > > > index d6466f96cdd0..357115ff0ec2 100644 > > > --- a/include/linux/virtio_net.h > > > +++ b/include/linux/virtio_net.h > > > @@ -5,7 +5,11 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include > > > +#include > > > +#include > > > +#include > > > #include > > > #include > > > #include > > > @@ -48,6 +52,171 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb, > > > return 0; > > > } > > > > > > +/* TODO: nothing virtio specific here, move to ip.h */ > > > + > > > +/* > > > + * Pull and minimally validate the IPv4 header at @nhoff. > > > + * Returns the header and stores the transport header offset in > > > + * @thoff, or NULL if truncated or malformed. Only the fixed header > > > + * is guaranteed linear; callers that need the options or the > > > + * transport header must pull further. > > > + */ > > > > nit: comment basically describes the code (what), which is quite > > self explanatory. I'd drop. Or else consider converting to kdoc fmt. > > > > > +static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb, > > > + unsigned int nhoff, > > > + unsigned int *thoff) > > > +{ > > > + struct iphdr *iph; > > > + > > > + if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr))) > > > + return NULL; > > > + > > > + iph = (struct iphdr *)(skb->data + nhoff); > > > + if (iph->version != 4 || > > > + iph->ihl < sizeof(struct iphdr) / sizeof(u32)) > > > + return NULL; > > > + > > > + *thoff = nhoff + iph->ihl * sizeof(u32); > > > + return iph; > > > +} > > > + > > > +/* Where does the L3 header start, and what protocol is it? > > > + * Handles Ethernet and VLAN headers; other devices use the > > > + * network header offset the caller set up. > > > + */ > > > +static inline bool > > > +virtio_net_l3_start(struct sk_buff *skb, int *nhoff, __be16 *proto) > > > +{ > > > + if (skb->dev->type == ARPHRD_ETHER) { > > > > Only since recent commit 44378d02c5aa ("net: always dissect GSO > > packets in __virtio_net_hdr_to_skb()") can skb->dev be relied on to be > > non-zero. > > > > We'll have to be careful that this does not get backported to stable > > without that. > > > > Perhaps an explicit precondition test and error return (and > > DEBUG_NET_WARN_ON_ONCE) if a NULL skb->dev is encountered. > > > > OTOH, it results in a hard crash so will be discoverd fast without > > that too. > > > > > > > + if (!pskb_may_pull(skb, ETH_HLEN)) > > > + return false; > > > + *nhoff = ETH_HLEN; > > > + *proto = eth_hdr(skb)->h_proto; > > > > When called from virtio_net_hdr_tnl_to_skb, according to my LLM mac > > header may not be set yet. Have not verified yet. > > > > skb_reset_mac_header(skb) is called inside __virtio_net_hdr_to_skb, > > which is then not called yet. > > > > I am able to confirm this in my testing. I have KASAN enabled, and it > triggered when testing leading me to this change. > > Adding `skb_reset_mac_header(skb);` before calling virtio_net_tnl_offsets_ok > mitigates this issue. > > --chris I was sure I run that test. Facepalm. Thanks a lot, I'll go over this again. > > > > + if (eth_type_vlan(*proto)) { > > > + *proto = __vlan_get_protocol(skb, *proto, nhoff); > > > + if (!*proto) > > > + return false; > > > + } > > > + } else { > > > + *nhoff = skb_network_offset(skb); > > > > The same is true for skb->network_header. Since an uninitialized/zero > > value there causes more subtle issues, here an explicit check may be > > more valuable (even) than for skb->dev. > > > > > + *proto = skb->protocol; > > > + } > > > + return true; > > > +} > > > + > > > > > > > > /* This function must be called after virtio_net_hdr_from_skb(). */ > > > @@ -282,6 +470,35 @@ static inline unsigned int virtio_l3min(bool is_ipv6) > > > return is_ipv6 ? sizeof(struct ipv6hdr) : sizeof(struct iphdr); > > > } > > > > > > +/* Do the tunnel offsets match the outer headers in the frame? > > > + * The L3 header the tunnel claims must be the one the frame > > > + * carries, with UDP following it at @outer_th. > > > + */ > > > +static inline bool > > > +virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6, > > > + unsigned int outer_th) > > > +{ > > > + int nhoff; > > > + __be16 proto; > > > + unsigned int thoff; > > > + int ip_proto; > > > + > > > + if (!virtio_net_l3_start(skb, &nhoff, &proto)) > > > + return false; > > > + if (outer_isv6 != (proto == htons(ETH_P_IPV6))) > > > + return false; > > > + if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto)) > > > + return false; > > > + > > > + return ip_proto == IPPROTO_UDP && outer_th == thoff; > > > +} > > > + > > > +/* > > > + * Return value: > > > + * 0 - success > > > + * >0 - checksum was completed in software (slow path) > > > + * <0 - failure > > > + */ > > > static inline int > > > virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > > > const struct virtio_net_hdr_v1_hash_tunnel *vhdr, > > > @@ -293,6 +510,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > > > unsigned int inner_nh, outer_th, inner_th; > > > unsigned int inner_l3min, outer_l3min; > > > u8 gso_inner_type, gso_tunnel_type; > > > + __be16 inner_proto; > > > bool outer_isv6, inner_isv6; > > > int ret; > > > > > > @@ -332,10 +550,13 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > > > inner_th < inner_nh + inner_l3min) > > > return -EINVAL; > > > > > > + if (!virtio_net_tnl_offsets_ok(skb, outer_isv6, outer_th)) > > > + return -EINVAL; > > > + > > > /* Let the basic parsing deal with plain GSO features. */ > > > ret = __virtio_net_hdr_to_skb(skb, hdr, true, > > > hdr->gso_type & ~gso_tunnel_type); > > > - if (ret) > > > + if (ret < 0) > > > return ret; > > > > > > /* In case of USO, the inner protocol is still unknown and > > > @@ -353,7 +574,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > > > } > > > > > > skb_set_inner_protocol(skb, inner_isv6 ? htons(ETH_P_IPV6) : > > > - htons(ETH_P_IP)); > > > + htons(ETH_P_IP)); > > > if (hdr->flags & VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM) { > > > if (!tnl_csum_negotiated) > > > return -EINVAL; > > > @@ -368,7 +589,17 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > > > skb->inner_mac_header = inner_nh + skb_headroom(skb); > > > skb->transport_header = outer_th + skb_headroom(skb); > > > skb->encapsulation = 1; > > > - return 0; > > > + > > > + /* > > > + * We know VIRTIO_NET_HDR_F_NEEDS_CSUM was set so __virtio_net_hdr_to_skb > > > + * set CHECKSUM_PARTIAL. > > > + */ > > > + inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP); > > > + if (virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, inner_nh, > > > + inner_proto)) > > > + return 0; > > > + > > > + return virtio_net_resolve_csum_partial_mismatch(skb); > > > } > > > > > > /* Checksum-related fields validation for the driver */ > > > -- > > > MST > > > > > > >