netdev - RE: [PATCH net 2/2] net/tls: handle errors from padding

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Date:   Fri, 10 May 2019 07:16:44 -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,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Dave Watson <davejwatson@...com>,
        Dirk van der Merwe <dirk.vandermerwe@...ronome.com>
Subject: RE: [PATCH net 2/2] net/tls: handle errors from padding_length()

Jakub Kicinski wrote:
> At the time padding_length() is called the record header
> is still part of the message.  If malicious TLS 1.3 peer
> sends an all-zero record padding_length() will stop at
> the record header, and return full length of the data
> including the tail_size.
> 
> Subsequent subtraction of prot->overhead_size from rxm->full_len
> will cause rxm->full_len to turn negative.  skb accessors,
> however, will always catch resulting out-of-bounds operation,
> so in practice this fix comes down to returning the correct
> error code.  It also fixes a set but not used warning.

In practice returning incorrect error codes to users can confuse
applications though so this seems important. I've observed apps
hang with wrong codes for example and this error seems to be pushed
all the way up to sw_recvmsg.

> 
> This code was added by commit 130b392c6cd6 ("net: tls: Add tls 1.3 support").
> 
> CC: Dave Watson <davejwatson@...com>
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@...ronome.com>
> ---

Looks good to me but one question below,

>  	/* After using skb->sk to propagate sk through crypto async callback
> @@ -1478,7 +1488,7 @@ static int decrypt_skb_update(struct sock *sk, struct sk_buff *skb,
>  	struct tls_prot_info *prot = &tls_ctx->prot_info;
>  	int version = prot->version;
>  	struct strp_msg *rxm = strp_msg(skb);
> -	int err = 0;
> +	int pad, err = 0;
>  
>  	if (!ctx->decrypted) {
>  #ifdef CONFIG_TLS_DEVICE
> @@ -1501,7 +1511,11 @@ static int decrypt_skb_update(struct sock *sk, struct sk_buff *skb,
>  			*zc = false;
>  		}
>  
> -		rxm->full_len -= padding_length(ctx, tls_ctx, skb);
> +		pad = padding_length(ctx, prot, skb);
> +		if (pad < 0)
> +			return pad;
> +

Need to review a bit closer on my side, but do we need to do any cleanup
if this fails? It looks like the other padding_length call sites will
but here we eventually return directly to recvmsg.

> +		rxm->full_len -= pad;
>  		rxm->offset += prot->prepend_size;
>  		rxm->full_len -= prot->overhead_size;
>  		tls_advance_record_sn(sk, &tls_ctx->rx, version);
> -- 
> 2.21.0
>