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

From: Tom Parkin <[email-masked]>
Date: Wed, 6 Mar 2024 13:14:23 +0000

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?
-- 
Tom Parkin
Katalix Systems Ltd
https://katalix.com
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