Re: [PATCH net v2 6/8] tcp: fix use-after-free in the lockless listener path

[Paolo Abeni]

On 9/1/26 9:37 AM, Hyunwoo Kim wrote:
> On Mon, Aug 24, 2026 at 12:32:50PM +0900, Hyunwoo Kim wrote:
>> tcp_v{4,6}_rcv() calls tcp_v{4,6}_do_rcv() without holding the socket
>> lock when sk->sk_state is TCP_LISTEN. Every other path into
>> tcp_v{4,6}_do_rcv() holds it.
>>
>> tcp_v{4,6}_do_rcv() and tcp_rcv_state_process() below it read
>> sk->sk_state again. A listener can leave TCP_LISTEN through
>> connect(AF_UNSPEC), and if that happens in between, the second read
>> returns a different state.
>>
>> tcp_rcv_established() or tcp_rcv_state_process() then runs without the
>> lock. If the second read returns TCP_SYN_SENT, the incoming SYN is
>> treated as a crossed SYN and reaches tcp_send_synack(). When the SYN skb
>> at the head of the retransmit queue is skb_cloned(), that function
>> replaces it with a copy and releases the original with
>> tcp_rtx_queue_unlink_and_free().
>>
>> The original is the skb that a thread on another CPU is transmitting
>> right now in __tcp_transmit_skb(). skb_cloned() is true because the
>> clone made for that transmit is still alive. Once the transmit returns,
>> tcp_update_skb_after_send() calls list_move_tail() on the skb's
>> tcp_tsorted_anchor.
>>
>> In short:
>>
>>   socket(AF_INET) -> bind() -> listen()   // the socket that changes state
>>   socket(AF_INET) -> bind() -> listen()   // the peer
>>
>>   Several threads keep opening new sockets and connecting to the first
>>   socket's address.
>>
>>   Another thread repeats this on the first socket:
>>     connect(AF_UNSPEC)      // TCP_LISTEN -> TCP_CLOSE
>>     connect(peer address)   // TCP_CLOSE -> TCP_SYN_SENT
>>                             // another CPU still sees a listener, handles
>>                             // one of those SYNs without the lock and
>>                             // releases the SYN skb that this connect()
>>                             // is transmitting
>>                             // -> use-after-free
>>     connect(AF_UNSPEC)
>>     listen()                // TCP_LISTEN again
>>
>> KASAN log:
>>
>>   BUG: KASAN: slab-use-after-free in __list_del_entry_valid_or_report+0x14/0x140
>>   Read of size 8 at addr ffff88800a5d1460 by task poc/125
>>   ...
>>   Call Trace:
>>    __list_del_entry_valid_or_report+0x14/0x140
>>    tcp_update_skb_after_send+0x62/0x170
>>    __tcp_transmit_skb+0xe33/0x1e40
>>    tcp_connect+0x1b67/0x2490
>>    tcp_v4_connect+0x998/0xab0
>>    __inet_stream_connect+0x22c/0x700
>>    inet_stream_connect+0x48/0x70
>>    __sys_connect+0x101/0x130
>>   ...
>>   Allocated by task 125:
>>    __alloc_skb+0xd1/0x370
>>    tcp_stream_alloc_skb+0x2d/0x2b0
>>    tcp_connect+0x72d/0x2490
>>    tcp_v4_connect+0x998/0xab0
>>    __inet_stream_connect+0x22c/0x700
>>    inet_stream_connect+0x48/0x70
>>    __sys_connect+0x101/0x130
>>   ...
>>   The buggy address belongs to the object at ffff88800a5d1400
>>    which belongs to the cache skbuff_fclone_cache of size 472
>>
>> Instead of taking the lock, keep the lockless path from reading
>> sk->sk_state again to decide how to process the packet. Move the
>> TCP_LISTEN handling out of tcp_rcv_state_process() into
>> tcp_rcv_listen_state_process(), and let the TCP_LISTEN branch of
>> tcp_v{4,6}_rcv() call a new tcp_v{4,6}_rcv_listen(). Listener processing
>> does not change. The TCP_LISTEN arm of tcp_v{4,6}_do_rcv() is left
>> alone, because a socket can finish listen() after the state check and a
>> backlogged skb is then processed there.
>>
>> Fixes: e994b2f0fb92 ("tcp: do not lock listener to process SYN packets")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
> 
> Looking at this further, unhashing the listener and then calling
> synchronize_net() lets the disconnect path handle it. MPTCP needs a fix
> too, though, because it closes and reuses the first subflow directly
> without going through tcp_disconnect().

This looks like a more palatable approach: this patch in the current
format looked way too invasive to me.

> This also closes the trigger path for patches 4, 5 and 8. I would still
> keep those, since they add no work to the fast path and they remove the
> root cause itself. Their changelogs would have to change though.

I'm unsure accepting new connections is not fast-path: the connection
per second rate is a relevant metric for a sever, even if the additional
cleanup is possibly not visible in most benchmarks.

Still I would avoid additional unneeded patches.

Thanks,

Paolo