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[35.245.46.111]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5e91cab4a97sm12928147e0c.6.2026.10.14.19.49.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 14 Oct 2026 19:49:14 -0700 (PDT) Date: Wed, 14 Oct 2026 22:49:13 -0400 From: Willem de Bruijn To: Boyan Liu , Willem de Bruijn , "David S . 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 Message-ID: In-Reply-To: <20261010073015.7665-1-yymhvert@gmail.com> References: <20261010073015.7665-1-yymhvert@gmail.com> Subject: Re: [PATCH net] udp: Fix out-of-bounds read with unset mac header in unexpected GSO path Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Boyan Liu wrote: > 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. I wonder if the right fix here is to not call skb_unset_mac_header in the first place. There are plenty of L3 device type with no L2 header. They still have a valid skb_mac_header. This fix may be papering over a tunnel doing something non-standard. > 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 >