linux-kernel - Re: [PATCH net] dccp: check for ccid in ccid_hc_tx_send

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Date:   Mon, 30 Oct 2023 17:24:22 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Bragatheswaran Manickavel <bragathemanick0908@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        dccp@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+c71bc336c5061153b502@...kaller.appspotmail.com
Subject: Re: [PATCH net] dccp: check for ccid in ccid_hc_tx_send_packet

On Mon, Oct 30, 2023 at 5:22 PM Bragatheswaran Manickavel
<bragathemanick0908@...il.com> wrote:
>
>
> On 30/10/23 21:19, Eric Dumazet wrote:
>
> On Mon, Oct 30, 2023 at 4:40 PM Bragatheswaran Manickavel
> <bragathemanick0908@...il.com> wrote:
>
> On 30/10/23 14:29, Eric Dumazet wrote:
>
> On Sat, Oct 28, 2023 at 4:41 PM Bragatheswaran Manickavel
> <bragathemanick0908@...il.com> wrote:
>
> ccid_hc_tx_send_packet might be called with a NULL ccid pointer
> leading to a NULL pointer dereference
>
> Below mentioned commit has similarly changes
> commit 276bdb82dedb ("dccp: check ccid before dereferencing")
>
> Reported-by: syzbot+c71bc336c5061153b502@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=c71bc336c5061153b502
> Signed-off-by: Bragatheswaran Manickavel <bragathemanick0908@...il.com>
> ---
>   net/dccp/ccid.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/dccp/ccid.h b/net/dccp/ccid.h
> index 105f3734dadb..1015dc2b9392 100644
> --- a/net/dccp/ccid.h
> +++ b/net/dccp/ccid.h
> @@ -163,7 +163,7 @@ static inline int ccid_packet_dequeue_eval(const int return_code)
>   static inline int ccid_hc_tx_send_packet(struct ccid *ccid, struct sock *sk,
>                                           struct sk_buff *skb)
>   {
> -       if (ccid->ccid_ops->ccid_hc_tx_send_packet != NULL)
> +       if (ccid != NULL && ccid->ccid_ops->ccid_hc_tx_send_packet != NULL)
>                  return ccid->ccid_ops->ccid_hc_tx_send_packet(sk, skb);
>          return CCID_PACKET_SEND_AT_ONCE;
>   }
> --
> 2.34.1
>
> If you are willing to fix dccp, I would make sure that some of
> lockless accesses to dccps_hc_tx_ccid
> are also double checked and fixed.
>
> do_dccp_getsockopt() and dccp_get_info()
>
> Hi Eric,
>
> In both do_dccp_getsockopt() and dccp_get_info(), dccps_hc_rx_ccid are
> checked properly before access.
>
> Not really, because another thread can change the value at the same time.
>
> Adding checks is not solving races.
>
> Understood. But when I see function similar to ccid_hc_tx_send_packet all of
> them has ccid check and few of them have addressed same issue.
>
> dccp_get_info()
>         if (dp->dccps_hc_rx_ccid != NULL)
>                 ccid_hc_rx_get_info(dp->dccps_hc_rx_ccid, sk, info);
>         if (dp->dccps_hc_tx_ccid != NULL)
>                 ccid_hc_tx_get_info(dp->dccps_hc_tx_ccid, sk, info);
>

All I am saying is that these changes are not correct.

They are simply adding some 'checks' that are unsafe.

Compiler can absolutely fetch dp->dccps_hc_tx_ccid a second time,
and a NULL could be read this second time.

> do_dccp_getsockopt()
>     ccid_hc_rx_getsockopt
>     ccid_hc_tx_getsockopt
>     ccid_get_current_rx_ccid
>     ccid_get_current_tx_ccid   ===> All of them have ccid check
>
> So, I went on with this changes.
> If you have another suggestion of fixing this issue please let me know. I will take a look.