netdev - RE: [PATCH net] net/tls: fix sk_msg trim on fallback to copy mode

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Date:   Thu, 31 Oct 2019 15:05:53 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, oss-drivers@...ronome.com,
        borisp@...lanox.com, aviadye@...lanox.com,
        john.fastabend@...il.com, daniel@...earbox.net,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        syzbot+f8495bff23a879a6d0bd@...kaller.appspotmail.com,
        syzbot+6f50c99e8f6194bf363f@...kaller.appspotmail.com,
        Eric Biggers <ebiggers@...nel.org>,
        herbert@...dor.apana.org.au, glider@...gle.com,
        linux-crypto@...r.kernel.org
Subject: RE: [PATCH net] net/tls: fix sk_msg trim on fallback to copy mode

Jakub Kicinski wrote:
> sk_msg_trim() tries to only update curr pointer if it falls into
> the trimmed region. The logic, however, does not take into the
> account pointer wrapping that sk_msg_iter_var_prev() does.
> This means that when the message was trimmed completely, the new
> curr pointer would have the value of MAX_MSG_FRAGS - 1, which is
> neither smaller than any other value, nor would it actually be
> correct.
> 
> Special case the trimming to 0 length a little bit.
> 
> This bug caused the TLS code to not copy all of the message, if
> zero copy filled in fewer sg entries than memcopy would need.
> 
> Big thanks to Alexander Potapenko for the non-KMSAN reproducer.
> 
> Fixes: d829e9c4112b ("tls: convert to generic sk_msg interface")
> Reported-by: syzbot+f8495bff23a879a6d0bd@...kaller.appspotmail.com
> Reported-by: syzbot+6f50c99e8f6194bf363f@...kaller.appspotmail.com
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> ---
> Daniel, John, does this look okay?

Thanks for the second ping!

> 
> CC: Eric Biggers <ebiggers@...nel.org>
> CC: herbert@...dor.apana.org.au
> CC: glider@...gle.com
> CC: linux-crypto@...r.kernel.org
> 
>  net/core/skmsg.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index cf390e0aa73d..c42c145216b1 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -276,7 +276,10 @@ void sk_msg_trim(struct sock *sk, struct sk_msg *msg, int len)
>  	 * However trimed data that has not yet been used in a copy op
>  	 * does not require an update.
>  	 */
> -	if (msg->sg.curr >= i) {
> +	if (!msg->sg.size) {
> +		msg->sg.curr = 0;
> +		msg->sg.copybreak = 0;
> +	} else if (msg->sg.curr >= i) {
>  		msg->sg.curr = i;
>  		msg->sg.copybreak = msg->sg.data[i].length;
>  	}
> -- 


Its actually not sufficient. We can't directly do comparisons against curr
like this. msg->sg is a ring buffer so we have to be careful for these
types of comparisons.

Examples hopefully help explian. Consider the case with a ring layout on
entering sk_msg_trim,

   0 1 2                              N = MAX_MSG_FRAGS
  |_|_|_|...|_|_|_|...|_|_|_|_|....|_|_|
       ^       ^         ^
       curr    end       start

Start trimming from end

   0 1 2                              N = MAX_MSG_FRAGS
  |X|X|X|...|X|X|_|...|_|_|i|X|....|X|X|
       ^       ^         ^
       curr    end       start

We trim backwards through ring with sk_msg_iter_var_prev(). And its
possible to end with the result of above where 'i' is greater than curr
and greater than start leaving scatterlist elements so size != 0.

    i > curr && i > start && sg.size != 0

but we wont catch it with this condition

    if (msg->sg.curr >= i)

So we won't reset curr and copybreak so we have a potential issue now
where curr is pointing at data that has been trimmed.

I'll put together a fix but the correct thing to do here is a proper
ring greater than op which is not what we have there. Although, your patch
is also really a good one to have because reseting curr = 0 and
copybreak = 0 when possible keeps the ring from being fragmented which
avoids chaining when we push scatterlists down to crypto layer. So for
your patch,

Acked-By: John Fastabend <john.fastabend@...il.com>

If it should go to net or net-next I think is probably up for debate

Nice catch!!! Can you send me the reproducer?

Thanks,
John