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[35.245.46.111]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5eae983efe8sm472945e0c.17.2026.10.20.12.44.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 12:44:36 -0700 (PDT) Date: Tue, 20 Oct 2026 15:44:35 -0400 From: Willem de Bruijn To: "Michael S. Tsirkin" , linux-kernel@vger.kernel.org Cc: Willem de Bruijn , Chris Arges , =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , Phil Sutter , 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, Max Krasnyansky , Rusty Russell , Paulos Yibelo Message-ID: In-Reply-To: <9c08d12b3a7819ae03ded6a154e4f908276e93d4.1792411304.git.mst@redhat.com> References: <9c08d12b3a7819ae03ded6a154e4f908276e93d4.1792411304.git.mst@redhat.com> Subject: Re: [PATCH net v3 5/8] net: resolve uncommon partial checksums at ingress X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list 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 | 272 ++++++++++++++++++++++++++++++++++++- > 1 file changed, 268 insertions(+), 4 deletions(-) > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h > index 013c6c7e2dea..97b554ef7123 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 +52,198 @@ 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. > + */ > +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) > +{ > + /* skb->dev is only guaranteed to be set since 44378d02c5aa > + * ("net: always dissect GSO packets in > + * __virtio_net_hdr_to_skb()"), do not crash when it is not. > + */ > + if (WARN_ON_ONCE(!skb->dev)) > + return false; > + > + if (skb->dev->type == ARPHRD_ETHER) { > + if (!pskb_may_pull(skb, ETH_HLEN)) > + return false; > + *nhoff = ETH_HLEN; > + *proto = eth_hdr(skb)->h_proto; > + if (eth_type_vlan(*proto)) { > + *proto = __vlan_get_protocol(skb, *proto, nhoff); > + if (!*proto) > + return false; > + } > + } else { loopback uses ARPHRD_LOOPBACK, but expects Ethernet (apparently). We can add that ARPHRD to the if condition. > + *nhoff = skb_network_offset(skb); > + /* An unset network header shows up as an offset > + * outside of the packet. > + */ > + if (*nhoff < 0 || (unsigned int)*nhoff >= skb->len) > + return false; > + *proto = skb->protocol; > + if (!*proto) { > + u8 version; > + > + /* A packet socket bound to ETH_P_ALL leaves the > + * protocol unset: infer it from the IP version > + * nibble at the network header. > + */ > + if (!pskb_may_pull(skb, *nhoff + 1)) > + return false; > + version = skb->data[*nhoff] >> 4; is this endian specific? maybe just cast to iphdr > + if (version == 4) > + *proto = htons(ETH_P_IP); > + else if (version == 6) > + *proto = htons(ETH_P_IPV6); > + } > + } > + return true; > +} > + > +/* Walk the L3 header at @nhoff: where does its transport header > + * start, and which protocol is it? Returns false if the header > + * is truncated, malformed or fragmented. > + */ > +static inline bool > +virtio_net_l4_start(struct sk_buff *skb, unsigned int nhoff, __be16 proto, > + unsigned int *thoff, int *ip_proto) > +{ > + if (proto == htons(ETH_P_IP)) { > + const struct iphdr *iph = virtio_net_ip4_parse(skb, nhoff, thoff); > + > + if (!iph || ip_is_fragment(iph)) > + return false; > + *ip_proto = iph->protocol; > + return true; > + } > + > + if (proto == htons(ETH_P_IPV6)) { > + const struct ipv6hdr *iph; > + int flags = 0; > + > + if (!pskb_may_pull(skb, nhoff + sizeof(*iph))) > + return false; > + iph = (const void *)(skb->data + nhoff); > + if (iph->version != 6) > + return false; > + *thoff = nhoff + sizeof(*iph); > + *ip_proto = iph->nexthdr; > + if (*ip_proto != IPPROTO_TCP && *ip_proto != IPPROTO_UDP) { > + *thoff = nhoff; > + *ip_proto = ipv6_find_hdr(skb, thoff, -1, NULL, > + &flags); > + if (*ip_proto < 0 || flags & IP6_FH_F_FRAG) > + return false; > + } > + return true; > + } > + > + return false; > +} for all this parsing: could we use flow dissection, like the !VIRTIO_NET_HDR_F_NEEDS_CSUM case? > + > +/* Does checksum metadata match a known TCP or UDP header? */ > +static inline bool > +virtio_net_csum_partial_ok_at_offset(struct sk_buff *skb, u8 hdr_gso_type, > + unsigned int nhoff, __be16 proto) > +{ > + unsigned int thoff; > + int ip_proto; > + > + if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto)) > + return false; > + > + if (skb_checksum_start_offset(skb) != thoff) > + return false; > + > + switch (ip_proto) { > + case IPPROTO_TCP: > + if (skb->csum_offset != offsetof(struct tcphdr, check) || > + !pskb_may_pull(skb, thoff + sizeof(struct tcphdr))) > + return false; > + break; > + case IPPROTO_UDP: > + if (skb->csum_offset != offsetof(struct udphdr, check) || > + !pskb_may_pull(skb, thoff + sizeof(struct udphdr))) > + return false; > + break; > + default: > + return false; > + } > + > + switch (hdr_gso_type & ~VIRTIO_NET_HDR_GSO_ECN) { > + case VIRTIO_NET_HDR_GSO_NONE: > + return true; > + case VIRTIO_NET_HDR_GSO_TCPV4: > + return proto == htons(ETH_P_IP) && ip_proto == IPPROTO_TCP; > + case VIRTIO_NET_HDR_GSO_TCPV6: > + return proto == htons(ETH_P_IPV6) && ip_proto == IPPROTO_TCP; > + case VIRTIO_NET_HDR_GSO_UDP: > + case VIRTIO_NET_HDR_GSO_UDP_L4: > + return ip_proto == IPPROTO_UDP; > + default: > + return false; > + } > +} > + > +static inline bool > +virtio_net_csum_partial_ok(struct sk_buff *skb, u8 hdr_gso_type) > +{ > + int nhoff; > + __be16 proto; > + > + if (!virtio_net_l3_start(skb, &nhoff, &proto)) > + return false; > + > + return virtio_net_csum_partial_ok_at_offset(skb, hdr_gso_type, nhoff, > + proto); > +} > + > +/* > + * Handle a packet whose checksum metadata does not match the packet: > + * GSO packets are dropped, as segmentation needs one checksum per > + * segment; others are completed in software. > + * > + * Return 1 if the checksum was completed in software, else a negative > + * errno. > + */ > +static inline int virtio_net_resolve_csum_partial_mismatch(struct sk_buff *skb) > +{ > + if (skb_is_gso(skb)) > + return -EINVAL; > + > + return skb_checksum_help(skb) ?: 1; > +} > + > static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb, > const struct virtio_net_hdr *hdr, > bool little_endian, u8 hdr_gso_type) > @@ -194,11 +390,36 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb, > return 0; > } > > +/* > + * Return value: > + * 0 - success > + * >0 - checksum was completed in software (slow path) > + * <0 - failure > + */ > static inline int virtio_net_hdr_to_skb(struct sk_buff *skb, > const struct virtio_net_hdr *hdr, > bool little_endian) > { > - return __virtio_net_hdr_to_skb(skb, hdr, little_endian, hdr->gso_type); > + typeof(skb->transport_header) old_transport = skb->transport_header; > + int ret = __virtio_net_hdr_to_skb(skb, hdr, little_endian, > + hdr->gso_type); > + > + if (ret < 0) > + return ret; > + > + if (skb->ip_summed != CHECKSUM_PARTIAL || > + virtio_net_csum_partial_ok(skb, hdr->gso_type)) > + return 0; > + > + /* > + * Here we do not trust the checksum metadata. > + * __virtio_net_hdr_to_skb set the transport header to csum_start > + * from virtio hdr, restore it to a known good value. > + */ > + skb->transport_header = old_transport; > + ret = virtio_net_resolve_csum_partial_mismatch(skb); > + > + return ret; can return virtio_net_resolve_csum_partial_mismatch(skb) > } > > /* This function must be called after virtio_net_hdr_from_skb(). */ > @@ -282,6 +503,35 @@ 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 +543,7 @@ 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; > > @@ -335,9 +586,12 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > /* 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; > > + if (!virtio_net_tnl_offsets_ok(skb, outer_isv6, outer_th)) > + return -EINVAL; > + > /* In case of USO, the inner protocol is still unknown and > * `inner_isv6` is just a guess, additional parsing is needed. > * The previous validation ensures that accessing an ipv4 inner > @@ -353,7 +607,7 @@ 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)); unintentional indentation change? > if (hdr->flags & VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM) { > if (!tnl_csum_negotiated) > return -EINVAL; > @@ -368,7 +622,17 @@ 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); GSO is implied when reaching this far, so can just return -EINVAL. And as only one caller of the function than remains, can avoid the helper. > } > > /* Checksum-related fields validation for the driver */ > -- > MST >