net: use skb_header_pointer() for TCPv4 GSO frag_off check

Overview

In the Linux kernel, the following vulnerability has been resolved:

net: use skb_header_pointer() for TCPv4 GSO frag_off check

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

Affected

• Linux >= cbc53e08a793b073e79f42ca33f1f3568703540d < 5057502630fbc8ce468ff617ff59af275960b537
• Linux >= cbc53e08a793b073e79f42ca33f1f3568703540d < f7a6cd508e9e825a2c69fa9e13d41ee156852f25
• Linux >= cbc53e08a793b073e79f42ca33f1f3568703540d < cc91202fc20a44aab4c206f12a2bfe05da936051
• Linux >= cbc53e08a793b073e79f42ca33f1f3568703540d < d970341cfa5594614c7a6634886c7688b4f5cafd
• Linux >= cbc53e08a793b073e79f42ca33f1f3568703540d < ddc748a391dd8642ba6b2e4fe22e7f2ddf84b7f0
• Linux 4.7

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.