Linux vulnerability

What happened

In the Linux kernel, the following vulnerability has been resolved: netfilter: synproxy: refresh tcphdr after skb_ensure_writable synproxy_tstamp_adjust() rewrites the TCP timestamp option in place and then patches the TCP checksum via inet_proto_csum_replace4() on the caller-supplied tcphdr pointer. Both ipv4_synproxy_hook() and ipv6_synproxy_hook() obtain that pointer with skb_header_pointer() before calling in, so it may either alias skb->head directly or point at the caller's on-stack _tcph buffer. Between obtaining the pointer and using it, the function calls skb_ensure_writable(skb, optend), which on a cloned or non-linear skb invokes pskb_expand_head() and frees the old skb->head. After that point the cached th is stale: caller (ipv[46]_synproxy_hook) th = skb_header_pointer(skb, ..., &_tcph) synproxy_tstamp_adjust(skb, protoff, th, ...) skb_ensure_writable(skb, optend) pskb_expand_head() /* kfree(old skb->head) */ ... inet_proto_csum_replace4(&th->check, ...) /* writes into freed head, or into the caller's stack copy leaving the on-wire checksum stale */ The option bytes are written through skb->data and are fine; only the checksum update goes through th and so lands in the wrong place. The result is either a write into freed slab memory or a packet leaving with a checksum that does not match its payload. Fix by re-deriving th from skb->data + protoff immediately after skb_ensure_writable() succeeds, so the subsequent checksum update targets the linear, writable header.

Affected versions

Linux: 48b1de4c110a7afa4b85862f6c75af817db26fad through before 9902a1058992de5d95656b64a3bd95c077f7ba2c (git); 48b1de4c110a7afa4b85862f6c75af817db26fad through before d3019c61799adc21811af4b521f11f3dc77f8e04 (git); 48b1de4c110a7afa4b85862f6c75af817db26fad through before dd206819f210522579010d889d45a9530bb494bc (git); 48b1de4c110a7afa4b85862f6c75af817db26fad through before af2c22ccb1f621aff487ff47a040e38e058541e7 (git); 48b1de4c110a7afa4b85862f6c75af817db26fad through before c7f945f7da097245a2f8ed7775ce48421047ee96 (git); 48b1de4c110a7afa4b85862f6c75af817db26fad through before f0fea2b6d5453a11ad11713bbf37561b9b3a7edf (git); 48b1de4c110a7afa4b85862f6c75af817db26fad through before a91887a5b6ee4b98dfbf1db657ed2b879430149e (git); 48b1de4c110a7afa4b85862f6c75af817db26fad through before 92170e6afe927ab2792a3f71902845789c8e31b1 (git); 3.12 Fixed: See vendor advisory.

Why it matters

Review the vendor advisory and exposure of the affected product to determine operational impact.

Detection & mitigation

• Apply vendor-provided updates or mitigations.
• Review affected product exposure and access logs.