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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, co+98d7de09cd94fec5@bugs.sh, Boyan Liu , stable@vger.kernel.org Subject: [PATCH net] udp: Fix out-of-bounds read with unset mac header in unexpected GSO path Date: Sat, 10 Oct 2026 15:30:15 +0800 Message-ID: <20261010073015.7665-1-yymhvert@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit udp_queue_rcv_skb() and udpv6_queue_rcv_skb() re-segment an unexpected GSO packet after restoring skb->data to the mac header with __skb_push(skb, -skb_mac_offset(skb)), assuming the mac header is valid. SRv6 End.DT4/End.DT6 VRF decapsulation calls skb_unset_mac_header(). iptunnel_pull_offloads() can leave an inner UDP GSO type (SKB_GSO_UDP_L4 or SKB_GSO_FRAGLIST) behind, so a socket that does not accept that GRO type runs the unexpected GSO path with mac_header == 0xffff. skb_mac_offset() then returns a large positive value, which the unsigned __skb_push() argument turns into a huge length: skb->data moves far outside the buffer and skb->len is corrupted, so segmentation reads out of bounds: BUG: KASAN: slab-out-of-bounds in inet_gso_segment (net/ipv4/af_inet.c:1383) Read of size 1 at addr ffff88800da7ffff by task exploit/149 Call Trace: inet_gso_segment (net/ipv4/af_inet.c:1383) skb_mac_gso_segment (net/core/gso.c:53) __skb_gso_segment (net/core/gso.c:124) udp_queue_rcv_skb (net/ipv4/udp.c:2476 net/ipv4/udp.c:2466) udp_unicast_rcv_skb (net/ipv4/udp.c:2620) ip_local_deliver (net/ipv4/ip_input.c:262) seg6_local_input_core (net/ipv6/seg6_local.c:1640) lwtunnel_input (net/core/lwtunnel.c:465) ipv6_rcv (net/ipv6/ip6_input.c:351) __skb_gso_segment() resets the mac header to wherever skb->data points, so the push target only needs to be a valid header start. Fall back to the network header when the mac header was never set. Fixes: cf329aa42b66 ("udp: cope with UDP GRO packet misdirection") Cc: stable@vger.kernel.org Reported-by: co+98d7de09cd94fec5@bugs.sh Signed-off-by: Boyan Liu --- net/ipv4/udp.c | 5 ++++- net/ipv6/udp.c | 5 ++++- 2 files changed, 8 insertions(+), 2 deletions(-) diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index bb8cfc62c..fe5a23f1c 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -2441,7 +2441,10 @@ static int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb) return udp_queue_rcv_one_skb(sk, skb); BUILD_BUG_ON(sizeof(struct udp_skb_cb) > SKB_GSO_CB_OFFSET); - __skb_push(skb, -skb_mac_offset(skb)); + if (skb_mac_header_was_set(skb)) + __skb_push(skb, -skb_mac_offset(skb)); + else + __skb_push(skb, -skb_network_offset(skb)); segs = udp_rcv_segment(sk, skb, true); skb_list_walk_safe(segs, skb, next) { __skb_pull(skb, skb_transport_offset(skb)); diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c index 93478d1ad..8db4e2869 100644 --- a/net/ipv6/udp.c +++ b/net/ipv6/udp.c @@ -895,7 +895,10 @@ static int udpv6_queue_rcv_skb(struct sock *sk, struct sk_buff *skb) if (likely(!udp_unexpected_gso(sk, skb))) return udpv6_queue_rcv_one_skb(sk, skb); - __skb_push(skb, -skb_mac_offset(skb)); + if (skb_mac_header_was_set(skb)) + __skb_push(skb, -skb_mac_offset(skb)); + else + __skb_push(skb, -skb_network_offset(skb)); segs = udp_rcv_segment(sk, skb, false); skb_list_walk_safe(segs, skb, next) { __skb_pull(skb, skb_transport_offset(skb)); -- 2.43.0