Re: [PATCH net 0/4] amt: fix relay tunnel keying and unauthenticated-Request DoS

[Omar Ramadan]

Thanks -- both are fair asks. Answers below; they are also reflected in
the v2 cover letter. v1 was generated against v7.1 and did not apply to
net, so this is answered against v2, which is posted against net with the
General Query patch dropped (that fix is already in net as afae89de73dd).
v2 is three patches.

How it was found
  Manual code inspection of the AMT relay path in drivers/net/amt.c, read
  against RFC 7450, with LLM assistance -- hence the Assisted-by: LLM
  trailer on patch 3/3. It was not a syzbot report or a static-analysis
  tool scan. Patch 1/3 (endpoint keying) came out of the same reading of
  amt_request_handler() and amt_update_handler() against RFC 7450 s4.2.2.

Whether it was triggered
  Found by inspection; not observed in production. These are availability
  and correctness defects, not memory-safety bugs, so there is no oops or
  stack trace to attach. The symptoms follow deterministically from the
  code the diffs change:

   - 3/3, exhaustion: amt_request_handler() allocated a tunnel before any
     validation of the source, so Relay Membership Requests from distinct
     spoofable source endpoints fill the table to max_tunnels (default
     128). Once full, further Requests are answered with ICMP_DEST_UNREACH
     to the (spoofed) source and genuine gateways are refused; re-sending
     once per amt_gmi() interval holds it full.
   - 3/3, desync: the pre-validation lookup jumped to the send path and
     overwrote an established tunnel's ->nonce/->mac, so a single spoofed
     Request carrying a known gateway's source endpoint made that
     gateway's next Membership Update fail the "Invalid MAC" check -- a
     silent one-packet denial that consumes no table slot.
   - 1/3, aliasing: address-only keying collapses two endpoints that share
     a source address (NAT, or one host using separate IPv4/IPv6 ports per
     RFC 7450 s4.2.2) onto one tunnel; the later Request wins and the
     earlier gateway silently stops receiving.

  I have not staged a live end-to-end exploit run -- the above is read
  from the code paths, not a captured trace.

Fix testing (this part is observed, not inferred)
  The three patches were applied to net and the kernel booted (arm64,
  QEMU via virtme-ng) with CONFIG_KASAN=y, CONFIG_PROVE_LOCKING=y,
  CONFIG_PROVE_RCU=y and CONFIG_DEBUG_LIST=y on top of
  tools/testing/selftests/net/config. tools/testing/selftests/net/amt.sh
  passes all six tests -- amt discovery, IPv4 and IPv6 multicast
  forwarding, and both IPv4/IPv6 traffic-forwarding torture cases all
  report [ OK ] -- with no KASAN, lockdep or RCU reports.