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[35.245.46.111]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5ea9909f364sm286539e0c.0.2026.10.15.10.43.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 15 Oct 2026 10:43:56 -0700 (PDT) Date: Thu, 15 Oct 2026 13:43:56 -0400 From: Willem de Bruijn To: "Michael S. Tsirkin" , linux-kernel@vger.kernel.org Cc: Willem de Bruijn , Jason Wang , =?UTF-8?B?RXVnZW5pbyBQw6lyZXo=?= , 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 , Chris Arges Message-ID: In-Reply-To: References: Subject: Re: [PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit 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. > + 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 >