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[35.245.46.111]) by smtp.gmail.com with ESMTPSA id ada2fe7eead31-7cc1928aef3sm7607647137.0.2026.10.18.07.16.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 18 Oct 2026 07:16:24 -0700 (PDT) Date: Sun, 18 Oct 2026 10:16:23 -0400 From: Willem de Bruijn To: "Michael S. Tsirkin" , linux-kernel@vger.kernel.org Cc: Willem de Bruijn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , netdev@vger.kernel.org Message-ID: In-Reply-To: References: Subject: Re: [PATCH net-next] net: gso: validate transport header before computing segment length 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 Michael S. Tsirkin wrote: > If a TCP GSO packet is processed in the stack before a transport TSO > header was ever established, the TCP data offset is read through the > unset header offset, far outside the packet. > > This was demonstrated by syzkaller, back when the input path did not > dissect such packets: a virtio net header supplying a malformed > network header was enough. Qdisc length estimation has carried a > guard against exactly this since a0dce8752193 ("net: Skip GSO > length estimation if transport header is not set"); the GSO length > validators never got one. > > We used to, but we no longer allow such packets in: since 44378d02c5aa > ("net: always dissect GSO packets in __virtio_net_hdr_to_skb()") the > virtio net input path dissects all GSO packets. > > But as a defence in depth measure, let's also check before we read: if > the TCP header is unset or outside the linear data, drop it. > > Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr") > Cc: Willem de Bruijn > Signed-off-by: Michael S. Tsirkin > --- > > Can be net or net-next, but given we have Eric's patch in net already, > net-next seems more appropriate, and it's safer. > > include/linux/skbuff.h | 5 +++++ > net/core/gso.c | 36 ++++++++++++++++++++++++++++++++++++ > 2 files changed, 41 insertions(+) > > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h > index a3ff380d43f0..aa5bc608de06 100644 > --- a/include/linux/skbuff.h > +++ b/include/linux/skbuff.h > @@ -3041,6 +3041,11 @@ static inline bool skb_inner_network_header_was_set(const struct sk_buff *skb) > return skb->inner_network_header > 0; > } > > +static inline bool skb_inner_transport_header_was_set(const struct sk_buff *skb) > +{ > + return skb->inner_transport_header > 0; skb_transport_header_was_set has return skb->transport_header != (typeof(skb->transport_header))~0U; is that initialization different for inner_transport_header? If so, slightly confusing. But also understandable that zero is the default initialization, I guess. > +} > + > static inline unsigned char *skb_inner_mac_header(const struct sk_buff *skb) > { > return skb->head + skb->inner_mac_header; > diff --git a/net/core/gso.c b/net/core/gso.c > index e96ef6350064..d98acfd68595 100644 > --- a/net/core/gso.c > +++ b/net/core/gso.c > @@ -131,6 +131,36 @@ struct sk_buff *__skb_gso_segment(struct sk_buff *skb, > } > EXPORT_SYMBOL(__skb_gso_segment); > > +/* > + * GSO processing (but not UDP/SCTP GSO, which uses fixed header TSO > + * sizes) reads the TCP header as specified by > + * skb->transport_header/inner_transport_header. Make sure it is > + * there to read. > + */ > +static bool skb_gso_transport_header_ok(const struct sk_buff *skb) > +{ > + const struct skb_shared_info *shinfo = skb_shinfo(skb); > + unsigned int thoff; > + > + if (likely(!(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)))) > + return true; The likely case for GSO is TSO. > + > + if (skb->encapsulation) { > + if (!skb_inner_transport_header_was_set(skb)) > + return false; > + > + thoff = skb_inner_transport_offset(skb); > + } else { > + if (!skb_transport_header_was_set(skb)) > + return false; > + > + thoff = skb_transport_offset(skb); > + } This is not GSO specific. Could be a helper in skbuff.h? > + > + /* A negative offset converts to a huge value and fails. */ > + return thoff + sizeof(struct tcphdr) <= skb_headlen(skb); > +} > + > /** > * skb_gso_transport_seglen - Return length of individual segments of a gso packet > * > @@ -253,6 +283,9 @@ static inline bool skb_gso_size_check(const struct sk_buff *skb, > */ > bool skb_gso_validate_network_len(const struct sk_buff *skb, unsigned int mtu) > { > + if (unlikely(!skb_gso_transport_header_ok(skb))) > + return false; > + seems weird to test a transport header feature in a network header test? Should this instead where it is needed? > return skb_gso_size_check(skb, skb_gso_network_seglen(skb), mtu); > } > EXPORT_SYMBOL_GPL(skb_gso_validate_network_len); > @@ -268,6 +301,9 @@ EXPORT_SYMBOL_GPL(skb_gso_validate_network_len); > */ > bool skb_gso_validate_mac_len(const struct sk_buff *skb, unsigned int len) > { > + if (unlikely(!skb_gso_transport_header_ok(skb))) > + return false; > + even more so in a link layer header test > return skb_gso_size_check(skb, skb_gso_mac_seglen(skb), len); > } > EXPORT_SYMBOL_GPL(skb_gso_validate_mac_len); > -- > MST >