[PATCH 6.12 061/242] net: use skb_header_pointer() for TCPv4 GSO frag_off check

Greg Kroah-Hartman
2026-04-08T18:41:54Z

From: Guoyu Su <yss2813483011xxl@gmail.com>

[ Upstream commit ddc748a391dd8642ba6b2e4fe22e7f2ddf84b7f0 ]

Syzbot reported a KMSAN uninit-value warning in gso_features_check()
called from netif_skb_features() [1].

gso_features_check() reads iph->frag_off to decide whether to clear
mangleid_features. Accessing the IPv4 header via ip_hdr()/inner_ip_hdr()
can rely on skb header offsets that are not always safe for direct
dereference on packets injected from PF_PACKET paths.

Use skb_header_pointer() for the TCPv4 frag_off check so the header read
is robust whether data is already linear or needs copying.

[1] https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407

Link: https://lore.kernel.org/netdev/willemdebruijn.kernel.1a9f35039caab@gmail.com/
Fixes: cbc53e08a793 ("GSO: Add GSO type for fixed IPv4 ID")
Reported-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407
Tested-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com
Signed-off-by: Guoyu Su <yss2813483011xxl@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260327153507.39742-1-yss2813483011xxl@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/core/dev.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 2748ee051bd1b..9c4f0e5f2c136 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3609,10 +3609,15 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
* IPv4 header has the potential to be fragmented.
*/
if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4) {
- struct iphdr *iph = skb->encapsulation ?
- inner_ip_hdr(skb) : ip_hdr(skb);
+ const struct iphdr *iph;
+ struct iphdr _iph;
+ int nhoff = skb->encapsulation ?
+ skb_inner_network_offset(skb) :
+ skb_network_offset(skb);

- if (!(iph->frag_off & htons(IP_DF)))
+ iph = skb_header_pointer(skb, nhoff, sizeof(_iph), &_iph);
+
+ if (!iph || !(iph->frag_off & htons(IP_DF)))
features &= ~dev->mangleid_features;
}

--
2.53.0