Subject: Re: [PATCH] Fix handling of verdicts after NF_QUEUE

From: Pablo Neira Ayuso <[email-masked]>
Date: Tue, 12 Dec 2017 01:23:18 +0100

Hi,

Thanks for catching up this, see below.

On Mon, Dec 11, 2017 at 06:30:24PM -0500, Debabrata Banerjee wrote:
> A verdict of NF_STOLEN after NF_QUEUE will cause an incorrect return value
> and a potential kernel panic via double free of skb's
> 
> This was broken by commit 7034b566a4e7 ("netfilter: fix nf_queue handling")
> and subsequently fixed in v4.10 by commit c63cbc460419 ("netfilter:
> use switch() to handle verdict cases from nf_hook_slow()"). However that
> commit cannot be cleanly cherry-picked to v4.9
> 
> Signed-off-by: Debabrata Banerjee <[email-masked]>
> 
> ---
> 
> This fix is only needed for v4.9 stable since v4.10+ does not have the
> issue
> ---
>  net/netfilter/core.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/netfilter/core.c b/net/netfilter/core.c
> index 004af030ef1a..d869ea50623e 100644
> --- a/net/netfilter/core.c
> +++ b/net/netfilter/core.c
> @@ -364,6 +364,11 @@ int nf_hook_slow(struct sk_buff *skb, struct nf_hook_state *state)
>  		ret = nf_queue(skb, state, &entry, verdict);
>  		if (ret == 1 && entry)
>  			goto next_hook;
> +	} else {
> +		/* Implicit handling for NF_STOLEN, as well as any other
> +		 * non conventional verdicts.
> +		 */
> +		ret = 0;

Another possibility (more simple?) would be this:

int nf_hook_slow(struct sk_buff *skb, struct nf_hook_state *state)
{
        struct nf_hook_entry *entry;
        unsigned int verdict;
-       int ret = 0;
+       int ret;

        entry = rcu_dereference(state->hook_entries);
next_hook:
+       ret = 0;

Basically, make sure ret is set to zero when jumping to the next_hook
label.

Thanks!