This series hardens AMT relay tunnel lifetime, validates gateway input, and
keeps unauthenticated Requests from consuming relay state. The problems
fixed by patches 2-4 were found during the AMT IPv6 outer transport
work, which is not included here.
I am using the existing IPv4 AMT relay and gateway implementation in a
multicast delivery network, where gateways on access networks send IGMP/MLD
membership state through a relay to reach downstream receivers.
The first patch closes the gc_wq re-arm use-after-free and makes teardown
safe for RCU readers and delayed group/source workers. The second rejects
Queries and Data from 0.0.0.0 before relay discovery and revalidates deferred
Queries. The third requires exactly four bytes for IPv4 netlink address
attributes. The fourth makes pre-Query drops visible, avoids counting
successful reports as drops, and uses concurrent-safe TX statistics. The
fifth keeps Requests stateless, authenticates Membership Updates before
allocation, keys state by address and port, bounds per-source admission, and
silently drops failed endpoint admission.
Each commit message now says how its problems were found. The problem in
patch 3 was triggered while writing an IPv6 selftest; the others were
found by review or code inspection.
My reply to Jakub on 2026-10-20 said I had reproduced the lifetime,
authentication, teardown and input-validation issues. That overstated
it: only the problem in patch 3 was triggered at runtime.
https://lore.kernel.org/netdev/20261020105410.7dccf739@kernel.org/ (local)
The 2026-10-20 posting was mislabelled v4 in its subjects; its changelog
already read "v5 changes". This posting is the next revision of it.
Changes since the 2026-10-20 posting:
- Patch 1: flush the deferred group/source GC with flush_delayed_work()
instead of cancel_delayed_work_sync(). Both GC works re-arm only
themselves, so cancelling them on tunnel expiry or device stop stopped
periodic GC for good after the first expiry. The flush only reclaims
promptly; lifetime rests on RCU and the cleared owner pointers, and the
comment and changelog now say so.
- Patch 1: clear snode->gnode and gnode->tunnel_list with WRITE_ONCE().
Neither pointer is __rcu, so RCU_INIT_POINTER() drew two new sparse
errors.
- Patch 1: keep amt_group_work()'s single out: exit and move
rcu_read_unlock() below the label, instead of open-coding
rcu_read_unlock() and dev_put() at three exits. Every exit still
drops RCU and then the timer's device reference.
- Patch 5: drop the paragraph about tunnel liveness checks, which moved
to patch 1 in the previous revision, and fix one continuation-line
alignment.
- Patch 5: copy the secret generations in the Update path under a
seqcount, amt->key_seq, rather than under amt->lock, as the Sashiko
AI review of the 2026-10-15 posting suggested. Forged Updates now
take amt->lock only on PREEMPT_RT, to wait for a writer. Requests
still read key[0], nr_tunnels and max_tunnels in one O(1) amt->lock
section.
- Patch 5: say in the commit message why one address may hold at most
16 tunnels.
- All: state how each issue was found, and put Signed-off-by above the
"---" separator (it was below it in the previous posting, so git am
dropped it).
Testing: on base-commit below, each patch builds without new warnings
with W=1 C=1 (sparse) for IPv6=y and IPv6=n.
With KASAN and lockdep enabled, tools/testing/selftests/net/amt.sh
passes (6/6) with no splats.
v4 (mislabelled, 2026-10-20): https://lore.kernel.org/netdev/20261020002557.3984927-1-omar@blockcast.net/ (local)
v4 (re-posted 2026-10-15): https://lore.kernel.org/netdev/20261015220648.1545576-1-omar@blockcast.net/ (local)
v4: https://lore.kernel.org/netdev/20261013175407.947606-1-omar@blockcast.net/ (local)
v3: https://lore.kernel.org/netdev/20261012125943.558629-1-omar@blockcast.net/ (local)
v2: https://lore.kernel.org/netdev/20261009201455.1904698-1-omar@blockcast.net/ (local)
v1: https://lore.kernel.org/netdev/20261008003606.3666617-1-omar@blockcast.net/ (local)
Assisted-by: LLM
Omar Ramadan (5):
amt: fix use-after-free when a Membership Update re-arms a freed
tunnel
amt: reject a gateway Query or Data from 0.0.0.0 before a relay is
learned
amt: require an exact IPv4-sized IFLA_AMT_{LOCAL,DISCOVERY}_IP
amt: make pre-query report drops visible
amt: harden relay authentication and teardown
drivers/net/amt.c | 550 +++++++++++++++++++++++++++++++++++-----------
include/net/amt.h | 33 ++-
2 files changed, 450 insertions(+), 133 deletions(-)
base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
--
2.47.3