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)
-
opt_flen is a 16-bit accumulator:
-
include/net/ipv6.h:298 defines __u16 opt_flen; /* after fragment hdr */.
-
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.
-
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.
-
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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