ipv6: validate extension header length before copying to cmsg

Overview

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.

Affected

• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 81394827dfb72772c50d0ae3bdfa094428a5d76d
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 931b4a1f13408c2507719890f78f7227c34a0282
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 08464413e628803bd10cb1df68d0138665f2f885
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < a29768d56eb3798c052ad3281b05596e695a17af
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 0d330eff318c0f44d4fb0ad2c2aef38f87f24c90
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < a35daeabb433686234b010ebf7b53778dbd6c9b8
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < eb18a1b1644e4cad978df2131e2bb9a2e6886992
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < dd433671fef381fdaf7b530c631e6b782d66e224
• Linux 2.6.12

Remediation

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