tcp: Avoid infinite loop on recvmsg bug

On Wed, 2012-11-07 at 15:33 -0800, Eric Dumazet wrote:

> So you probably are fighting a bug we already fixed in upstream kernel.
> 
> (commit c8628155ece363 "tcp: reduce out_of_order memory use" did not
> played well with cloned skbs.)
> 
> This issue was already discussed on netdev in the past.

If you use a 3.4 kernel, you want the following patch.

(I guess you could reproduce the crash easily running a tcpdump in //)





--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

> So you probably are fighting a bug we already fixed in upstream kernel.
>
> (commit c8628155ece363 "tcp: reduce out_of_order memory use" did not
> played well with cloned skbs.)
>
> This issue was already discussed on netdev in the past.

Thanks for the hint. Unfortunately, we have not pulled c8628 into our
tree yet, so that's not it. Is there another point where the cloned
skb or the faked truesize might make it break? We have been running
this test with that hardware some 30 times in the last months and only
seen it once, so it cannot be that common.

I have noticed that you have already proposed a patch to repair
smsc95xx (replacing the clone with a copy) on this list a few times...
what's the status on that? Will it be committed eventually or did you
abandon that approach?

Regardless of that, I still think that the bug handling in tcp_recvmsg
should be updated in one way or the other.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

On Wed, 2012-11-07 at 18:25 -0800, Julius Werner wrote:
> > So you probably are fighting a bug we already fixed in upstream kernel.
> >
> > (commit c8628155ece363 "tcp: reduce out_of_order memory use" did not
> > played well with cloned skbs.)
> >
> > This issue was already discussed on netdev in the past.
> 
> Thanks for the hint. Unfortunately, we have not pulled c8628 into our
> tree yet, so that's not it. Is there another point where the cloned
> skb or the faked truesize might make it break? We have been running
> this test with that hardware some 30 times in the last months and only
> seen it once, so it cannot be that common.

Update : Chrome OS current tree is based on 3.4 and really needed the
patch :

https://gerrit.chromium.org/gerrit/#/c/37666/



--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Hi Dave,

Have you thought about picking up one of the patches to tcp_recvmsg I
proposed in this thread? We consider the underlying bug in Chromium OS
that led mere here to be fixed now, but I bet this will not be the
last time someone hits this code path and has to deal with the bad
error handling.

I understand that not everyone here agrees on what the best solution
is, but I think both of them are far better than the inconsistent and
potentially hard-disk-filling way that the current kernel does it.

On Wed, 2012-11-07 at 11:33 -0800, Julius Werner wrote:
> tcp_recvmsg contains a sanity check that WARNs when there is a gap
> between the socket's copied_seq and the first buffer in the
> sk_receive_queue. In theory, the TCP stack makes sure that This Should
> Never Happen (TM)... however, practice shows that there are still a few
> bug reports from it out there (and one in my inbox).
>
> Unfortunately, when it does happen for whatever reason, the situation
> is not handled very well: the kernel logs a warning and breaks out of
> the loop that walks the receive queue. It proceeds to find nothing else
> to do on the socket and hits sk_wait_data, which cannot block because
> the receive queue is not empty. As no data was read, the outer while
> loop repeats (logging the same warning again) ad infinitum until the
> system's syslog exhausts all available hard drive capacity.
>
> This patch addresses that issue by closing the socket outright and
> throwing EBADFD to userspace (which seems most appropriate to me at this
> point). As the underlying bug condition is "impossible" and therefore by
> definition unrecoverable, this is the only sensible action other than a
> full panic.
>
> Signed-off-by: Julius Werner <jwerner@chromium.org>
> ---
>  net/ipv4/tcp.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 197c000..d612308 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1628,7 +1628,7 @@ int tcp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
>                                "recvmsg bug: copied %X seq %X rcvnxt %X fl %X\n",
>                                *seq, TCP_SKB_CB(skb)->seq, tp->rcv_nxt,
>                                flags))
> -                             break;
> +                             goto selfdestruct;
>
>                       offset = *seq - TCP_SKB_CB(skb)->seq;
>                       if (tcp_hdr(skb)->syn)
> @@ -1936,6 +1936,11 @@ recv_urg:
>  recv_sndq:
>       err = tcp_peek_sndq(sk, msg, len);
>       goto out;
> +
> +selfdestruct:
> +     err = -EBADFD;
> +     tcp_done(sk);
> +     goto out;
>  }
>  EXPORT_SYMBOL(tcp_recvmsg);
>

On Tue, Nov 06, 2012 at 04:15:35PM -0800, Julius Werner wrote:
> tcp_recvmsg contains a sanity check that WARNs when there is a gap
> between the socket's copied_seq and the first buffer in the
> sk_receive_queue. In theory, the TCP stack makes sure that This Should
> Never Happen (TM)... however, practice shows that there are still a few
> bug reports from it out there (and one in my inbox).
>
> Unfortunately, when it does happen for whatever reason, the situation
> is not handled very well: the kernel logs a warning and breaks out of
> the loop that walks the receive queue. It proceeds to find nothing else
> to do on the socket and hits sk_wait_data, which cannot block because
> the receive queue is not empty. As no data was read, the outer while
> loop repeats (logging the same warning again) ad infinitum until the
> system's syslog exhausts all available hard drive capacity.
>
> This patch improves that behavior by going straight to a proper kernel
> crash. The cause of the error can be identified right away and the
> system's hard drive is not unnecessarily strained.
>
> Signed-off-by: Julius Werner <jwerner@chromium.org>
> ---
>  net/ipv4/tcp.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 197c000..fcb0927 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1628,7 +1628,7 @@ int tcp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
>                               "recvmsg bug: copied %X seq %X rcvnxt %X fl %X\n",
>                               *seq, TCP_SKB_CB(skb)->seq, tp->rcv_nxt,
>                               flags))
> -                            break;
> +                            BUG();
>
>                      offset = *seq - TCP_SKB_CB(skb)->seq;
>                      if (tcp_hdr(skb)->syn)
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

I've tossed these two patches under the carpet, so you'll need to
repost whichever one you want me to consider.

Basically, discussing old patches is pretty useless without a resend
to get it back into the fore-front of the patchwork queue.  So please
don't reference old stale patches without an associated repost like
this.

Thanks.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html