Wind River Support Network

Description

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---