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Sun, 20 Sep 2026 19:53:51 -0700 (PDT) From: Paulos Yibelo To: netdev@vger.kernel.org Cc: richard@nod.at, anton.ivanov@cambridgegreys.com, johannes@sipsolutions.net, willemdebruijn.kernel@gmail.com, jasowangio@gmail.com, mst@redhat.com, eperezma@redhat.com, xuanzhuo@linux.alibaba.com, andrew+netdev@lunn.ch, pablo@netfilter.org, fw@strlen.de, phil@nwl.cc, razor@blackwall.org, idosch@nvidia.com, dsahern@kernel.org, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, linux-um@lists.infradead.org, virtualization@lists.linux.dev, netfilter-devel@vger.kernel.org, coreteam@netfilter.org, bridge@lists.linux.dev, linux-kernel@vger.kernel.org Subject: [PATCH net v5 2/2] ip: reject partial checksums covering network headers Date: Sun, 20 Sep 2026 22:53:41 -0400 Message-ID: <20260921025341.44846-3-habte.yibelo@gmail.com> X-Mailer: git-send-email 2.46.0 In-Reply-To: <20260921025341.44846-1-habte.yibelo@gmail.com> References: <20260920004733.6473-1-habte.yibelo@gmail.com> <20260921025341.44846-1-habte.yibelo@gmail.com> Precedence: bulk X-Mailing-List: bridge@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit ip_do_fragment() and nf_br_ip_fragment() complete a CHECKSUM_PARTIAL skb before reading the IPv4 header length. ip6_fragment() and br_ip6_fragment() complete one after parsing the IPv6 header chain. A virtualization interface can supply a checksum start which, after link-layer removal, still points inside that parsed network header. skb_checksum_help() then writes the completed checksum into header bytes the stack has already consumed. For IPv4, changing iph->ihl after routing and validation can make fragmentation copy beyond the skb's logical linear head into transmitted options. A negative checksum-start offset is rejected by skb_checksum_help(), but only after a WARN_ONCE which can panic a panic_on_warn system. Validate the checksum start against the parsed header length before completing it. For IPv4, read and validate IHL first, retain it, and reacquire iph after skb_checksum_help() in both implementations. For IPv6, use the length returned by ip6_find_1stfragopt() in both implementations. Compare the signed checksum-start offset with the bounded signed header length so integer promotion cannot bypass either boundary. Fixes: dbd3393c56a8 ("ipv4: add defensive check for CHECKSUM_PARTIAL skbs in ip_fragment") Fixes: 405c92f7a541 ("ipv6: add defensive check for CHECKSUM_PARTIAL skbs in ip_fragment") Fixes: 3c171f496ef5 ("netfilter: bridge: add connection tracking system") Fixes: 764dd163ac92 ("netfilter: nf_conntrack_bridge: add support for IPv6") Reported-by: Paulos Yibelo Link: https://lore.kernel.org/netdev/20260920004733.6473-3-habte.yibelo@gmail.com/ Cc: stable@vger.kernel.org Signed-off-by: Paulos Yibelo --- Changes in v5: - Compare the checksum-start offset and IPv4 header length as signed values. - Add parsed-header checks to the IPv4/IPv6 output and bridge-netfilter fragmentation paths. - Drop the prior Acked-by and Reviewed-by tags because the code changed. Changes in v4: - State that a TUN device is sufficient and no guest is required, as noted by Michael S. Tsirkin. Changes in v3: - No code changes. Changes in v2: - No code changes. net/bridge/netfilter/nf_conntrack_bridge.c | 21 +++++++++++++++----- net/ipv4/ip_output.c | 23 ++++++++++++++++------ net/ipv6/ip6_output.c | 12 ++++++++--- net/ipv6/netfilter.c | 12 ++++++++--- 4 files changed, 51 insertions(+), 17 deletions(-) diff --git a/net/bridge/netfilter/nf_conntrack_bridge.c b/net/bridge/netfilter/nf_conntrack_bridge.c index 7ecb8a26b..d81ed8692 100644 --- a/net/bridge/netfilter/nf_conntrack_bridge.c +++ b/net/bridge/netfilter/nf_conntrack_bridge.c @@ -38,18 +38,29 @@ static int nf_br_ip_fragment(struct net *net, struct sock *sk, struct iphdr *iph; int err = 0; - /* for offloaded checksums cleanup checksum before fragmentation */ - if (skb->ip_summed == CHECKSUM_PARTIAL && - (err = skb_checksum_help(skb))) + iph = ip_hdr(skb); + hlen = iph->ihl * 4; + if (unlikely(hlen < sizeof(*iph) || hlen > skb_headlen(skb))) { + err = -EINVAL; goto blackhole; + } - iph = ip_hdr(skb); + /* Complete offloaded checksums only after the validated IP header. */ + if (skb->ip_summed == CHECKSUM_PARTIAL) { + if (unlikely(skb_checksum_start_offset(skb) < (int)hlen)) { + err = -EINVAL; + goto blackhole; + } + err = skb_checksum_help(skb); + if (err) + goto blackhole; + iph = ip_hdr(skb); + } /* * Setup starting values */ - hlen = iph->ihl * 4; frag_max_size -= hlen; ll_rs = LL_RESERVED_SPACE(skb->dev); mtu = skb->dev->mtu; diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c index a24cc8ee1..fa6a74d20 100644 --- a/net/ipv4/ip_output.c +++ b/net/ipv4/ip_output.c @@ -770,16 +770,28 @@ int ip_do_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, struct ip_frag_state state; int err = 0; - /* for offloaded checksums cleanup checksum before fragmentation */ - if (skb->ip_summed == CHECKSUM_PARTIAL && - (err = skb_checksum_help(skb))) - goto fail; - /* * Point into the IP datagram header. */ iph = ip_hdr(skb); + hlen = iph->ihl * 4; + if (unlikely(hlen < sizeof(*iph) || hlen > skb_headlen(skb))) { + err = -EINVAL; + goto fail; + } + + /* Complete offloaded checksums only after the validated IP header. */ + if (skb->ip_summed == CHECKSUM_PARTIAL) { + if (unlikely(skb_checksum_start_offset(skb) < (int)hlen)) { + err = -EINVAL; + goto fail; + } + err = skb_checksum_help(skb); + if (err) + goto fail; + iph = ip_hdr(skb); + } mtu = ip_skb_dst_mtu(sk, skb); if (IPCB(skb)->frag_max_size && IPCB(skb)->frag_max_size < mtu) @@ -789,7 +801,6 @@ int ip_do_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, * Setup starting values. */ - hlen = iph->ihl * 4; if (mtu < hlen + 8) { err = -EMSGSIZE; goto fail; diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c index 550965058..d157b6ade 100644 --- a/net/ipv6/ip6_output.c +++ b/net/ipv6/ip6_output.c @@ -942,9 +942,15 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, frag_id = ipv6_select_ident(net, &ipv6_hdr(skb)->daddr, &ipv6_hdr(skb)->saddr); - if (skb->ip_summed == CHECKSUM_PARTIAL && - (err = skb_checksum_help(skb))) - goto fail; + if (skb->ip_summed == CHECKSUM_PARTIAL) { + if (unlikely(skb_checksum_start_offset(skb) < (int)hlen)) { + err = -EINVAL; + goto fail; + } + err = skb_checksum_help(skb); + if (err) + goto fail; + } prevhdr = skb_network_header(skb) + nexthdr_offset; hroom = LL_RESERVED_SPACE(rt->dst.dev); diff --git a/net/ipv6/netfilter.c b/net/ipv6/netfilter.c index a7025ec87..da7ada12f 100644 --- a/net/ipv6/netfilter.c +++ b/net/ipv6/netfilter.c @@ -144,9 +144,15 @@ int br_ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, frag_id = ipv6_select_ident(net, &ipv6_hdr(skb)->daddr, &ipv6_hdr(skb)->saddr); - if (skb->ip_summed == CHECKSUM_PARTIAL && - (err = skb_checksum_help(skb))) - goto blackhole; + if (skb->ip_summed == CHECKSUM_PARTIAL) { + if (unlikely(skb_checksum_start_offset(skb) < (int)hlen)) { + err = -EINVAL; + goto blackhole; + } + err = skb_checksum_help(skb); + if (err) + goto blackhole; + } prevhdr = skb_network_header(skb) + nexthdr_offset; hroom = LL_RESERVED_SPACE(skb->dev); -- 2.46.0