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

ipv6: avoid overflows in ip6_datagram_send_ctl()

Yiming Qian reported : <quote> I believe I found a locally triggerable kernel bug in the IPv6 sendmsg ancillary-data path that can panic the kernel via skb_under_panic() (local DoS).

The core issue is a mismatch between:

• a 16-bit length accumulator (struct ipv6_txoptions::opt_flen, type __u16) and
• a pointer to the last provided destination-options header (opt->dst1opt)

when multiple IPV6_DSTOPTS control messages (cmsgs) are provided.

• include/net/ipv6.h:
  • struct ipv6_txoptions::opt_flen is __u16 (wrap possible). (lines 291-307, especially 298)
• net/ipv6/datagram.c:ip6_datagram_send_ctl():
  • Accepts repeated IPV6_DSTOPTS and accumulates into opt_flen without rejecting duplicates. (lines 909-933)
• net/ipv6/ip6_output.c:__ip6_append_data():
  • Uses opt->opt_flen + opt->opt_nflen to compute header sizes/headroom decisions. (lines 1448-1466, especially 1463-1465)
• net/ipv6/ip6_output.c:__ip6_make_skb():
  • Calls ipv6_push_frag_opts() if opt->opt_flen is non-zero. (lines 1930-1934)
• net/ipv6/exthdrs.c:ipv6_push_frag_opts() / ipv6_push_exthdr():
  • Push size comes from ipv6_optlen(opt->dst1opt) (based on the pointed-to header). (lines 1179-1185 and 1206-1211)

1. opt_flen is a 16-bit accumulator:

• include/net/ipv6.h:298 defines __u16 opt_flen; /* after fragment hdr */.

2. ip6_datagram_send_ctl() accepts repeated IPV6_DSTOPTS cmsgs and increments opt_flen each time:

• In net/ipv6/datagram.c:909-933, for IPV6_DSTOPTS:
  • It computes len = ((hdr->hdrlen + 1) << 3);
  • It checks CAP_NET_RAW using ns_capable(net->user_ns, CAP_NET_RAW). (line 922)
  • Then it does:
    • opt->opt_flen += len; (line 927)
    • opt->dst1opt = hdr; (line 928)

There is no duplicate rejection here (unlike the legacy IPV6_2292DSTOPTS path which rejects duplicates at net/ipv6/datagram.c:901-904).

If enough large IPV6_DSTOPTS cmsgs are provided, opt_flen wraps while dst1opt still points to a large (2048-byte) destination-options header.

In the attached PoC (poc.c):

• 32 cmsgs with hdrlen=255 => len = (255+1)*8 = 2048
• 1 cmsg with hdrlen=0 => len = 8
• Total increment: 32*2048 + 8 = 65544, so (__u16)opt_flen == 8
• The last cmsg is 2048 bytes, so dst1opt points to a 2048-byte header.

3. The transmit path sizes headers using the wrapped opt_flen:

• In net/ipv6/ip6_output.c:1463-1465:
  • headersize = sizeof(struct ipv6hdr) + (opt ? opt->opt_flen + opt->opt_nflen : 0) + ...;

With wrapped opt_flen, headersize/headroom decisions underestimate what will be pushed later.

4. When building the final skb, the actual push length comes from dst1opt and is not limited by wrapped opt_flen:

• In net/ipv6/ip6_output.c:1930-1934:
  • if (opt->opt_flen) proto = ipv6_push_frag_opts(skb, opt, proto);
• In net/ipv6/exthdrs.c:1206-1211, ipv6_push_frag_opts() pushes dst1opt via ipv6_push_exthdr().
• In net/ipv6/exthdrs.c:1179-1184, ipv6_push_exthdr() does:
  • skb_push(skb, ipv6_optlen(opt));
  • memcpy(h, opt, ipv6_optlen(opt));

With insufficient headroom, skb_push() underflows and triggers skb_under_panic() -> BUG():

• net/core/skbuff.c:2669-2675 (skb_push() calls skb_under_panic())

• net/core/skbuff.c:207-214 (skb_panic() ends in BUG())

• The IPV6_DSTOPTS cmsg path requires CAP_NET_RAW in the target netns user namespace (ns_capable(net->user_ns, CAP_NET_RAW)).

• Root (or any task with CAP_NET_RAW) can trigger this without user namespaces.

• An unprivileged uid=1000 user can trigger this if unprivileged user namespaces are enabled and it can create a userns+netns to obtain namespaced CAP_NET_RAW (the attached PoC does this).

• Local denial of service: kernel BUG/panic (system crash).

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