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Sun, 08 Mar 2026 00:33:41 -0800 (PST) From: Guoyu Su To: edumazet@google.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com Cc: netdev@vger.kernel.org, horms@kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com, Guoyu Su Subject: [PATCH net v2] net: use skb_header_pointer() in gso_features_check() for TCPv4 GSO Date: Sun, 8 Mar 2026 16:33:19 +0800 Message-Id: <20260308083319.1255118-1-yss2813483011xxl@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260307162905.3697050-1-yss2813483011xxl@gmail.com> References: <20260307162905.3697050-1-yss2813483011xxl@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Syzbot reported a KMSAN uninit-value warning in netif_skb_features() [1], which originates from gso_features_check(): BUG: KMSAN: uninit-value in gso_features_check net/core/dev.c:3804 [inlin= e] BUG: KMSAN: uninit-value in netif_skb_features+0x6fb/0x1870 net/core/dev.= c:3828 gso_features_check net/core/dev.c:3804 [inline] netif_skb_features+0x6fb/0x1870 net/core/dev.c:3828 validate_xmit_skb+0xb6/0x2400 net/core/dev.c:4003 ... __dev_queue_xmit+0x3016/0x5a50 net/core/dev.c:4795 packet_snd net/packet/af_packet.c:3077 [inline] packet_sendmsg+0x91d9/0xa320 net/packet/af_packet.c:3109 The current code dereferences iph->frag_off directly to decide whether to clear mangleid_features. For malformed or truncated skb data, this can read invalid/uninitialized header bytes. Use skb_header_pointer() to read the IPv4 header safely from either the outer or inner network header offset. If the header is not accessible, conservatively clear mangleid_features. This keeps the original behavior for valid packets while fixing the unsafe header access pattern. [1] https://syzkaller.appspot.com/bug?extid=3D1543a7d954d9c6d00407 Fixes: cbc53e08a793 ("GSO: Add GSO type for fixed IPv4 ID") Reported-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/68e4b5d8.050a0220.256323.0018.GAE@googl= e.com/T/ Tested-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com Signed-off-by: Guoyu Su --- v2: - Drop the SKB_GSO_DODGY shortcut. - Use skb_header_pointer() for safe IPv4 header access. - Add missing Fixes tag as requested. v1: https://lore.kernel.org/netdev/20260307162905.3697050-1-yss2813483011xx= l@gmail.com/ net/core/dev.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/net/core/dev.c b/net/core/dev.c index 14a83f2035b9..c70263654680 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -3805,10 +3805,13 @@ static netdev_features_t gso_features_check(const s= truct sk_buff *skb, * segmentation-offloads.rst). */ if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4) { - struct iphdr *iph =3D skb->encapsulation ? - inner_ip_hdr(skb) : ip_hdr(skb); + struct iphdr _iph; + const struct iphdr *iph; + int nhoff =3D skb->encapsulation ? + skb_inner_network_offset(skb) : skb_network_offset(skb); =20 - if (!(iph->frag_off & htons(IP_DF))) + iph =3D skb_header_pointer(skb, nhoff, sizeof(_iph), &_iph); + if (!iph || !(iph->frag_off & htons(IP_DF))) features &=3D ~dev->mangleid_features; } =20 --=20 2.34.1