Re: [PATCH v6] tcp: fix tcp_init_transfer() to not reset icsk_ca_initialized

[Neal Cardwell]

On Mon, Jul 5, 2021 at 7:19 PM Nguyen Dinh Phi <phind.uet@xxxxxxxxx> wrote:
>
> This commit fixes a bug (found by syzkaller) that could cause spurious
> double-initializations for congestion control modules, which could cause
> memory leaks or other problems for congestion control modules (like CDG)
> that allocate memory in their init functions.
>
> The buggy scenario constructed by syzkaller was something like:
>
> (1) create a TCP socket
> (2) initiate a TFO connect via sendto()
> (3) while socket is in TCP_SYN_SENT, call setsockopt(TCP_CONGESTION),
>     which calls:
>        tcp_set_congestion_control() ->
>          tcp_reinit_congestion_control() ->
>            tcp_init_congestion_control()
> (4) receive ACK, connection is established, call tcp_init_transfer(),
>     set icsk_ca_initialized=0 (without first calling cc->release()),
>     call tcp_init_congestion_control() again.
>
> Note that in this sequence tcp_init_congestion_control() is called
> twice without a cc->release() call in between. Thus, for CC modules
> that allocate memory in their init() function, e.g, CDG, a memory leak
> may occur. The syzkaller tool managed to find a reproducer that
> triggered such a leak in CDG.
>
> The bug was introduced when that commit 8919a9b31eb4 ("tcp: Only init
> congestion control if not initialized already")
> introduced icsk_ca_initialized and set icsk_ca_initialized to 0 in
> tcp_init_transfer(), missing the possibility for a sequence like the
> one above, where a process could call setsockopt(TCP_CONGESTION) in
> state TCP_SYN_SENT (i.e. after the connect() or TFO open sendmsg()),
> which would call tcp_init_congestion_control(). It did not intend to
> reset any initialization that the user had already explicitly made;
> it just missed the possibility of that particular sequence (which
> syzkaller managed to find).
>
> Fixes: 8919a9b31eb4 ("tcp: Only init congestion control if not initialized already")
> Reported-by: syzbot+f1e24a0594d4e3a895d3@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Nguyen Dinh Phi <phind.uet@xxxxxxxxx>
> ---
> V2:     - Modify the Subject line.
>         - Adjust the commit message.
>         - Add Fixes: tag.
> V3:     - Fix netdev/verify_fixes format error.
> V4:     - Add blamed authors to receiver list.
> V5:     - Add comment about the congestion control initialization.
> V6:     - Fix typo in commit message.
>  net/ipv4/tcp_input.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 7d5e59f688de..84c70843b404 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5922,8 +5922,8 @@ void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
>                 tp->snd_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
>         tp->snd_cwnd_stamp = tcp_jiffies32;
>
> -       icsk->icsk_ca_initialized = 0;
>         bpf_skops_established(sk, bpf_op, skb);
> +       /* Initialize congestion control unless BPF initialized it already: */
>         if (!icsk->icsk_ca_initialized)
>                 tcp_init_congestion_control(sk);
>         tcp_init_buffer_space(sk);
> --

Acked-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Tested-by: Neal Cardwell <ncardwell@xxxxxxxxxx>

Looks good to me.  Thanks for the fix!

neal