Subject: Re: [PATCH net 1/3] netfilter: synproxy: Fix out of bounds when parsing TCP options

From: Florian Westphal <[email-masked]>
Date: Wed, 9 Jun 2021 16:51:15 +0200

Maxim Mikityanskiy <[email-masked]> wrote:
> The TCP option parser in synproxy (synproxy_parse_options) could read
> one byte out of bounds. When the length is 1, the execution flow gets
> into the loop, reads one byte of the opcode, and if the opcode is
> neither TCPOPT_EOL nor TCPOPT_NOP, it reads one more byte, which exceeds
> the length of 1.
> 
> This fix is inspired by commit 9609dad263f8 ("ipv4: tcp_input: fix stack
> out of bounds when parsing TCP options.").
> 
> Cc: Young Xiao <[email-masked]>
> Fixes: 48b1de4c110a ("netfilter: add SYNPROXY core/target")
> Signed-off-by: Maxim Mikityanskiy <[email-masked]>
> ---
>  net/netfilter/nf_synproxy_core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/netfilter/nf_synproxy_core.c b/net/netfilter/nf_synproxy_core.c
> index b100c04a0e43..621eb5ef9727 100644
> --- a/net/netfilter/nf_synproxy_core.c
> +++ b/net/netfilter/nf_synproxy_core.c
> @@ -47,6 +47,8 @@ synproxy_parse_options(const struct sk_buff *skb, unsigned int doff,
>  			length--;
>  			continue;
>  		default:
> +			if (length < 2)
> +				return true;

Would you mind a v2 that also rejects bogus th->doff value when
computing the length?

Thanks.