Fix handling of verdicts after NF_QUEUE

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 <dbanerje@akamai.com>

---

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(+)

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 <dbanerje@akamai.com>
> 
> ---
> 
> 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!
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> From: Pablo Neira Ayuso [mailto:pablo@netfilter.org]
> On Mon, Dec 11, 2017 at 06:30:24PM -0500, Debabrata Banerjee wrote:
> > +	} 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.

Many ways to fix it, but I thought including the comment was appropriate.
Happy to change it if we want simpler instead.

-Deb
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On Tue, Dec 12, 2017 at 12:36:35AM +0000, Banerjee, Debabrata wrote:
> > From: Pablo Neira Ayuso [mailto:pablo@netfilter.org]
> > On Mon, Dec 11, 2017 at 06:30:24PM -0500, Debabrata Banerjee wrote:
> > > +	} 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.
> 
> Many ways to fix it, but I thought including the comment was appropriate.
> Happy to change it if we want simpler instead.

OK, let's take this one.

Please, send a patch in git-format-patch, that we can pass to -stable.

Cc netfilter-devel@vger.kernel.org and stable@vger.kernel.org should
be fine, you can also include gregkh@linuxfoundation.org since he
maintains 4.9-stable.

I'll ack this by when you send it.

Thanks!
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