[PATCH net v2] ipv6: do not let ipv6_find_hdr() return an offset past the packet end

ipv6_find_hdr() walks the extension header chain, skipping each header by
the length that header itself declares.  ipv6_optlen() returns up to 2048,
and the skip is never checked against skb->len, so the offset stored in
*offset can point past the end of the packet.

openvswitch installs that offset as the transport header, and
update_ipv6_checksum() then reads and writes the transport checksum field
out of bounds:

  BUG: KASAN: slab-use-after-free in inet_proto_csum_replace16+0x445/0x470
  Read of size 2 at addr ffff88810b754b06 by task ovs_ipv6_oob/629
  CPU: 4 UID: 1000 PID: 629 Comm: ovs_ipv6_oob Tainted: G N 7.3.0-rc3+ #348
  Call Trace:
   inet_proto_csum_replace16+0x445/0x470
   set_ipv6_addr+0x3dd/0x460
   do_execute_actions+0x6a3d/0x7c40
   ovs_execute_actions+0xfd/0x480
   ovs_packet_cmd_execute+0xc38/0xf20
   genl_rcv_msg+0x59e/0x870
   netlink_rcv_skb+0x18b/0x450
   genl_rcv+0x2d/0x40
   netlink_unicast+0x6bc/0xa20

  The buggy address belongs to the object at ffff88810b754980
   which belongs to the cache skbuff_small_head of size 704
  The buggy address is located 390 bytes inside of
   freed 704-byte region [ffff88810b754980, ffff88810b754c40)

Other callers use that offset too, so bound it here rather than in one
caller.

Reject a header whose declared length does not fit in the packet.
ipv6_find_hdr() already fails with -EBADMSG on a malformed chain, so this
adds no new failure mode.

Fixes: f8f626754ebe ("ipv6: Move ipv6_find_hdr() out of Netfilter code.")
Suggested-by: Ilya Maximets <i.maximets@ovn.org>
Suggested-by: Eric Dumazet <edumazet@google.com>
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Norbert Szetei <redacted>
---
v2:
  - fix ipv6_find_hdr() instead of the openvswitch caller, per the review
    of v1
  - dropped the openvswitch change
  - retitled
v1: https://lore.kernel.org/netdev/114FAFBD-F7BC-46AE-AF9F-0D15124F130E@doyensec.com/ (local)

Reproducer available on request.

 net/ipv6/exthdrs_core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv6/exthdrs_core.c b/net/ipv6/exthdrs_core.c
index 9d06d487e8b1..4a9748338cf4 100644
--- a/net/ipv6/exthdrs_core.c
+++ b/net/ipv6/exthdrs_core.c

@@ -278,6 +278,9 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
 			hdrlen = ipv6_optlen(hp);
 
 		if (!found) {
+			if (skb->len - start < hdrlen)
+				return -EBADMSG;
+
 			nexthdr = hp->nexthdr;
 			start += hdrlen;
 		}

-- 
2.55.0