Re: [PATCH net-next] l2tp: fix incorrect parameter validation in the pppol2tp_getsockopt() function

[Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>]

On 3/6/24 16:14, Tom Parkin wrote:
> On  Wed, Mar 06, 2024 at 09:58:10 +0000, Gavrilov Ilia wrote:
>> diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
>> index f011af6601c9..6146e4e67bbb 100644
>> --- a/net/l2tp/l2tp_ppp.c
>> +++ b/net/l2tp/l2tp_ppp.c
>> @@ -1356,11 +1356,11 @@ static int pppol2tp_getsockopt(struct socket *sock, int level, int optname,
>>   	if (get_user(len, optlen))
>>   		return -EFAULT;
>>   
>> -	len = min_t(unsigned int, len, sizeof(int));
>> -
>>   	if (len < 0)
>>   		return -EINVAL;
>>   
>> +	len = min_t(unsigned int, len, sizeof(int));
>> +
>>   	err = -ENOTCONN;
>>   	if (!sk->sk_user_data)
>>   		goto end;
> 
> I think this code in l2tp_ppp.c has probably been inspired by a
> similar implementations elsewhere in the kernel -- for example
> net/ipv4/udp.c udp_lib_getsockopt does the same thing, and apparently
> has been that way since the dawn of git time.
> 
> I note however that plenty of other getsockopt implementations which
> are using min_t(unsigned int, len, sizeof(int)) don't check the length
> value at all: as an example, net/ipv6/raw.c do_rawv6_getsockopt.
> 
> As it stands right now in the l2tp_ppp.c code, I think the check on
> len will end up doing nothing, as you point out.
> 
> So moving the len check to before the min_t() call may in theory
> possibly catch out (insane?) userspace code passing in negative
> numbers which may "work" with the current kernel code.
> 
> I wonder whether its safer therefore to remove the len check
> altogether?

Thank you for answer.

In my opinion, it is better to leave the 'len' check. This way it will 
be easier for the user to understand where the error is.