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>
---
v7:
 - Drop revision-diff wording from commit message body.
 - Minor style: keep variable definitions contiguous in gso_features_check().
 - No functional change from v6.

v6: https://lore.kernel.org/netdev/20260326121813.457049-1-yss2813483011xxl@gmail.com/
v5: https://lore.kernel.org/netdev/20260320141459.9691-1-yss2813483011xxl@gmail.com/
v4: https://lore.kernel.org/netdev/20260319005421.14908-1-yss2813483011xxl@gmail.com/
v3: https://lore.kernel.org/netdev/20260312104351.185370-1-yss2813483011xxl@gmail.com/
v2: https://lore.kernel.org/netdev/20260308083319.1255118-1-yss2813483011xxl@gmail.com/
v1: https://lore.kernel.org/netdev/20260307162905.3697050-1-yss2813483011xxl@gmail.com/

 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 fc5557062414..831129f2a69b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3821,10 +3821,15 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
 	 * segmentation-offloads.rst).
 	 */
 	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;
 	}
 
-- 
2.34.1

Guoyu Su wrote:
> 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>

Sashiko asks about similar inner_ip_hdr in skb_gso_has_extension_hdr,
but userspace cannot set skb->encapsulation, so that's a false positive.