linux-kernel - Re: [PATCH] ipv6_sockglue: Fix a missing-check bug in ip6_ra

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Message-ID: <1b5f82ae-31a7-db36-dc9d-efc46cda2af3@suse.cz>
Date:   Mon, 1 Jul 2019 10:57:36 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Gen Zhang <blackgod016574@...il.com>, davem@...emloft.net,
        kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipv6_sockglue: Fix a missing-check bug in
 ip6_ra_control()

On 24. 05. 19, 5:19, Gen Zhang wrote:
> In function ip6_ra_control(), the pointer new_ra is allocated a memory 
> space via kmalloc(). And it is used in the following codes. However, 
> when there is a memory allocation error, kmalloc() fails. Thus null 
> pointer dereference may happen. And it will cause the kernel to crash. 
> Therefore, we should check the return value and handle the error.
> 
> Signed-off-by: Gen Zhang <blackgod016574@...il.com>
> 
> ---
> diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
> index 40f21fe..0a3d035 100644
> --- a/net/ipv6/ipv6_sockglue.c
> +++ b/net/ipv6/ipv6_sockglue.c
> @@ -68,6 +68,8 @@ int ip6_ra_control(struct sock *sk, int sel)
>  		return -ENOPROTOOPT;
>  
>  	new_ra = (sel >= 0) ? kmalloc(sizeof(*new_ra), GFP_KERNEL) : NULL;
> +	if (sel >= 0 && !new_ra)
> +		return -ENOMEM;
>  
>  	write_lock_bh(&ip6_ra_lock);
>  	for (rap = &ip6_ra_chain; (ra = *rap) != NULL; rap = &ra->next) {
> 

Was this really an omission? There is (!new_ra) handling below the for loop:
        if (!new_ra) {
                write_unlock_bh(&ip6_ra_lock);
                return -ENOBUFS;
        }

It used to handle both (sel >= 0) and (sel == 0) cases and it used to
return ENOBUFS in case of failure. For (sel >= 0) it also could at least
return EADDRINUSE when a collision was found -- even if memory was
exhausted.

In anyway, how could this lead to a pointer dereference? And why/how did
this get a CVE number?

thanks,
-- 
js
suse labs