CVE-2026-31415 ipv6: avoid overflows in ip6_datagram_send_ctl()
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).
 -
---truncated---
CVSS v3.1 score: 5.5