CVE-2026-64114: ipv4: raw: reject IP_HDRINCL packets with ihl < 5

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

ipv4: raw: reject IPHDRINCL packets with ihl < 5

rawsendhdrinc() validates that the caller-supplied IPv4 header fits within the message length:

iphlen = iph->ihl 4; err = -EINVAL; if (iphlen > length) goto errorfree;

if (iphlen >= sizeof(iph)) { / fix up saddr, totlen, id, csum, transportheader / }

It does not, however, reject ihl < 5. For such a packet the "if (iphlen >= sizeof(iph))" branch is skipped, leaving the crafted iphdr untouched, but the packet is still handed to iplocalout() and onward. Downstream consumers that read iph->ihl assume a sane value: net/ipv4/ah4.c:ahoutput() in particular subtracts sizeof(struct iphdr) from topiph->ihl 4 and passes the (signed-int-negative, then cast to sizet) result to memcpy(), producing an OOB access of length close to SIZEMAX and a host kernel panic.

An IPv4 header with ihl < 5 is malformed by definition (RFC 791: "Internet Header Length is the length of the internet header in 32 bit words ... Note that the minimum value for a correct header is 5."). The kernel should not be willing to inject such a packet into its own output path.

Reject "iphlen < sizeof(iph)" alongside the existing "iphlen > length" check. This matches the principle that locally constructed packets that re-enter the IP stack must pass the same basic sanity tests that a foreign packet would be subjected to.

Once this lands, the "if (iphlen >= sizeof(iph))" wrapper around the fixup branch becomes redundant; left in place to keep the patch minimal and backport-friendly. A follow-up can unwrap it.

Note that commit 86f4c90a1c5c ("ipv4, ipv6: ensure raw socket message is big enough to hold an IP header") ensures the message buffer is large enough to hold an iphdr, but does not constrain the self-reported iph->ihl.

Reachability: the malformed packet source is any caller with CAPNETRAW, including an unprivileged process in a user+net namespace on a kernel with CONFIGUSERNS=y. The reproduced AH crash also requires a matching xfrm AH policy on the outgoing route; a container granted CAPNETADMIN can install that state and policy in its netns. Loopback bypasses xfrmoutput, so the trigger uses a real netdev.

Reproduced on UML + KASAN: kernel-mode fault at addr 0x0 with memcpyorig at the crash site. Same shape reproduces inside a rootless Docker container with --cap-add NETADMIN on a stock distro kernel.