CVE-2026-63920

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

ipv6: validate extension header length before copying to cmsg

ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}

cmsgs (and their IPV6_2292* legacy counterparts) by trusting the

on-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length.

The length was validated only at parse time (ipv6_parse_hopopts(),

etc.). An nftables payload-write expression can rewrite hdrlen after

parsing and before the skb reaches recvmsg; the write itself is

in-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040

bytes from an 8-byte header. nftables is reachable from an

unprivileged user namespace, so this is an unprivileged

slab-out-of-bounds read:

BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540

put_cmsg+0x3ac/0x540

udpv6_recvmsg+0xca0/0x1250

sock_recvmsg+0xdf/0x190

____sys_recvmsg+0x1b1/0x620

Add ipv6_get_exthdr_len() which validates that at least two bytes

are accessible before reading the hdrlen field, then checks the

computed length against skb_tail_pointer(skb), returning 0 on

failure. Extension headers are kept in the linear skb area by

pskb_may_pull() during input, so skb_tail_pointer() is the correct

bound.

Use ipv6_get_exthdr_len() at all non-AH call sites: the five

standalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR)

and the three standard cases in the extension-header walk loop

(DSTOPTS, ROUTING, default). AH retains an inline bounds check

because its length formula differs ((ptr[1]+2)<<2).

The walk loop also gets a pre-read bounds check at the top to

validate ptr before any case accesses ptr[0] or ptr[1].

When the walk loop detects a corrupted header, return from the

function instead of continuing to process later socket options.