Re: [PATCH RFC v2] rxrpc: Fix use-after-free in rxrpc_destroy_all_peers()

Can we please add some code comments (1-3 lines max, properly
formatted) explaining why the rcu_barrier() and the spin_lock are
needed respectively? Having all this in the commit message is fine,
but will get lost.

On Tue, 18 Aug 2026 at 18:27, syzbot <[email protected]> wrote:
>
> During network namespace teardown, rxrpc_destroy_all_peers() iterates over
> the rxnet->peer_hash table to print leaked peers. However, it does so
> without holding rxnet->peer_hash_lock. This allows a race condition with
> asynchronous peer destruction, where RCU callbacks concurrently remove
> peers from the hash table and free them. When rxrpc_destroy_all_peers()
> accesses the freed peer, it results in a KASAN slab-use-after-free.
>
> BUG: KASAN: slab-use-after-free in rxrpc_destroy_all_peers+0xcc/0x150
> net/rxrpc/peer_object.c:461
> Read of size 8 at addr ffff88811089e420 by task kworker/u8:1/13
> Call Trace:
>  <TASK>
>  rxrpc_destroy_all_peers+0xcc/0x150 net/rxrpc/peer_object.c:461
>  rxrpc_exit_net+0x7f/0xc0 net/rxrpc/net_ns.c:114
>  ops_exit_list net/core/net_namespace.c:199 [inline]
>  ops_undo_list+0x43d/0x8d0 net/core/net_namespace.c:252
>  cleanup_net+0x572/0x810 net/core/net_namespace.c:702
>  process_one_work kernel/workqueue.c:3322 [inline]
>  process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
>  worker_thread+0x92d/0xe10 kernel/workqueue.c:3486
>  kthread+0x388/0x470 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
>
> To fix the slab-use-after-free, acquire rxnet->peer_hash_lock with
> spin_lock_bh() while iterating over rxnet->peer_hash in
> rxrpc_destroy_all_peers().
>
> Additionally, insert rcu_barrier() in rxrpc_exit_net() prior to destroying
> peers and local endpoints. Without rcu_barrier(), rxrpc_destroy_all_peers()
> races with pending RCU callbacks that drop peer and local references,
> logging spurious leak warnings. Furthermore, this missing synchronization
> causes a downstream panic hazard in rxrpc_destroy_all_locals() because
> in-flight peer releases leave rxnet->local_endpoints populated. Calling
> rcu_barrier() ensures all pending RCU callbacks that drop peer and local
> references complete prior to leak checks and local endpoint destruction.
>
> Fixes: 17226f124038 ("rxrpc: Fix leak of rxrpc_peer objects")
> Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
> Reported-by: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=c876adfab6362679008c
> Link: https://syzkaller.appspot.com/ai_job?id=ca098ef4-2067-4197-977f-d5d409f27cb8
> To: "David S. Miller" <[email protected]>
> To: "David Howells" <[email protected]>
> To: "Eric Dumazet" <[email protected]>
> To: "Jakub Kicinski" <[email protected]>
> To: <[email protected]>
> To: "Marc Dionne" <[email protected]>
> To: <[email protected]>
> To: "Paolo Abeni" <[email protected]>
> Cc: "Simon Horman" <[email protected]>
> Cc: <[email protected]>
>
> ---
> v2:
> - Added rcu_barrier() in rxrpc_exit_net() before peer and local endpoint teardown.
> - Updated commit message to detail how rcu_barrier() prevents spurious leak warnings and downstream panics in rxrpc_destroy_all_locals().
>
> v1:
> https://lore.kernel.org/all/[email protected]/T/
> ---
> diff --git a/net/rxrpc/net_ns.c b/net/rxrpc/net_ns.c
> index 9a9834145..6895f51b7 100644
> --- a/net/rxrpc/net_ns.c
> +++ b/net/rxrpc/net_ns.c
> @@ -111,6 +111,7 @@ static __net_exit void rxrpc_exit_net(struct net *net)
>         timer_delete_sync(&rxnet->peer_keepalive_timer);
>         rxrpc_destroy_all_calls(rxnet);
>         rxrpc_destroy_all_connections(rxnet);
> +       rcu_barrier();
>         rxrpc_destroy_all_peers(rxnet);
>         rxrpc_destroy_all_locals(rxnet);
>         proc_remove(rxnet->proc_net);
> diff --git a/net/rxrpc/peer_object.c b/net/rxrpc/peer_object.c
> index fa9a406e1..d81ed180e 100644
> --- a/net/rxrpc/peer_object.c
> +++ b/net/rxrpc/peer_object.c
> @@ -454,6 +454,8 @@ void rxrpc_destroy_all_peers(struct rxrpc_net *rxnet)
>         struct rxrpc_peer *peer;
>         int i;
>
> +       spin_lock_bh(&rxnet->peer_hash_lock);
> +
>         for (i = 0; i < HASH_SIZE(rxnet->peer_hash); i++) {
>                 if (hlist_empty(&rxnet->peer_hash[i]))
>                         continue;
> @@ -465,6 +467,8 @@ void rxrpc_destroy_all_peers(struct rxrpc_net *rxnet)
>                                &peer->srx.transport);
>                 }
>         }
> +
> +       spin_unlock_bh(&rxnet->peer_hash_lock);
>  }
>
>  /**
>
>
> base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
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