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

[Maxim Mikityanskiy <maximmi@nvidia.com>]

On 2021-06-09 17:51, Florian Westphal wrote:
> Maxim Mikityanskiy <maximmi@nvidia.com> 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 <92siuyang@gmail.com>
>> Fixes: 48b1de4c110a ("netfilter: add SYNPROXY core/target")
>> Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
>> ---
>>   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?

Could you elaborate? The length is a signed int calculated as `(th->doff 
* 4) - sizeof(*th)`. Invalid doff values (0..4) lead to negative length, 
so we never enter the loop. Or are you concerned of passing a negative 
length to skb_header_pointer?

> 
> Thanks.
>