[PATCH] netfilter: cttimeout: remove VLA usage

[PATCH] netfilter: cttimeout: remove VLA usage

[Gustavo A. R. Silva]

In preparation to enabling -Wvla, remove VLA and replace it
with dynamic memory allocation.

>From a security viewpoint, the use of Variable Length Arrays can be
a vector for stack overflow attacks. Also, in general, as the code
evolves it is easy to lose track of how big a VLA can get. Thus, we
can end up having segfaults that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 net/netfilter/nfnetlink_cttimeout.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/net/netfilter/nfnetlink_cttimeout.c b/net/netfilter/nfnetlink_cttimeout.c
index 6819300..dcd7bd3 100644
--- a/net/netfilter/nfnetlink_cttimeout.c
+++ b/net/netfilter/nfnetlink_cttimeout.c
@@ -51,19 +51,27 @@ ctnl_timeout_parse_policy(void *timeouts,
 			  const struct nf_conntrack_l4proto *l4proto,
 			  struct net *net, const struct nlattr *attr)
 {
+	struct nlattr **tb;
 	int ret = 0;
 
-	if (likely(l4proto->ctnl_timeout.nlattr_to_obj)) {
-		struct nlattr *tb[l4proto->ctnl_timeout.nlattr_max+1];
+	if (!l4proto->ctnl_timeout.nlattr_to_obj)
+		return 0;
 
-		ret = nla_parse_nested(tb, l4proto->ctnl_timeout.nlattr_max,
-				       attr, l4proto->ctnl_timeout.nla_policy,
-				       NULL);
-		if (ret < 0)
-			return ret;
+	tb = kcalloc(l4proto->ctnl_timeout.nlattr_max + 1, sizeof(*tb),
+		     GFP_KERNEL);
 
-		ret = l4proto->ctnl_timeout.nlattr_to_obj(tb, net, timeouts);
-	}
+	if (!tb)
+		return -ENOMEM;
+
+	ret = nla_parse_nested(tb, l4proto->ctnl_timeout.nlattr_max, attr,
+			       l4proto->ctnl_timeout.nla_policy, NULL);
+	if (ret < 0)
+		goto err;
+
+	ret = l4proto->ctnl_timeout.nlattr_to_obj(tb, net, timeouts);
+
+err:
+	kfree(tb);
 	return ret;
 }
 
-- 
2.7.4

Re: [PATCH] netfilter: cttimeout: remove VLA usage

[Joe Perches]

On Mon, 2018-03-12 at 18:14 -0500, Gustavo A. R. Silva wrote:
> In preparation to enabling -Wvla, remove VLA and replace it
> with dynamic memory allocation.
> 
> From a security viewpoint, the use of Variable Length Arrays can be
> a vector for stack overflow attacks. Also, in general, as the code
> evolves it is easy to lose track of how big a VLA can get. Thus, we
> can end up having segfaults that are hard to debug.
> 
> Also, fixed as part of the directive to remove all VLAs from
[]
> diff --git a/net/netfilter/nfnetlink_cttimeout.c b/net/netfilter/nfnetlink_cttimeout.c
[]
> @@ -51,19 +51,27 @@ ctnl_timeout_parse_policy(void *timeouts,
>  			  const struct nf_conntrack_l4proto *l4proto,
>  			  struct net *net, const struct nlattr *attr)
>  {
> +	struct nlattr **tb;
>  	int ret = 0;
>  
> -	if (likely(l4proto->ctnl_timeout.nlattr_to_obj)) {
> -		struct nlattr *tb[l4proto->ctnl_timeout.nlattr_max+1];
> +	if (!l4proto->ctnl_timeout.nlattr_to_obj)
> +		return 0;

Why not
	if unlikely(!...)
	
>  
> -		ret = nla_parse_nested(tb, l4proto->ctnl_timeout.nlattr_max,
> -				       attr, l4proto->ctnl_timeout.nla_policy,
> -				       NULL);
> -		if (ret < 0)
> -			return ret;
> +	tb = kcalloc(l4proto->ctnl_timeout.nlattr_max + 1, sizeof(*tb),
> +		     GFP_KERNEL);

kmalloc_array?

>  
> -		ret = l4proto->ctnl_timeout.nlattr_to_obj(tb, net, timeouts);
> -	}
> +	if (!tb)
> +		return -ENOMEM;
> +
> +	ret = nla_parse_nested(tb, l4proto->ctnl_timeout.nlattr_max, attr,
> +			       l4proto->ctnl_timeout.nla_policy, NULL);
> +	if (ret < 0)
> +		goto err;
> +
> +	ret = l4proto->ctnl_timeout.nlattr_to_obj(tb, net, timeouts);
> +
> +err:
> +	kfree(tb);
>  	return ret;
>  }
>  

Re: [PATCH] netfilter: cttimeout: remove VLA usage

[Pablo Neira Ayuso]

On Mon, Mar 12, 2018 at 04:58:38PM -0700, Joe Perches wrote:
> On Mon, 2018-03-12 at 18:14 -0500, Gustavo A. R. Silva wrote:
> > In preparation to enabling -Wvla, remove VLA and replace it
> > with dynamic memory allocation.
> > 
> > From a security viewpoint, the use of Variable Length Arrays can be
> > a vector for stack overflow attacks. Also, in general, as the code
> > evolves it is easy to lose track of how big a VLA can get. Thus, we
> > can end up having segfaults that are hard to debug.
> > 
> > Also, fixed as part of the directive to remove all VLAs from
> []
> > diff --git a/net/netfilter/nfnetlink_cttimeout.c b/net/netfilter/nfnetlink_cttimeout.c
> []
> > @@ -51,19 +51,27 @@ ctnl_timeout_parse_policy(void *timeouts,
> >  			  const struct nf_conntrack_l4proto *l4proto,
> >  			  struct net *net, const struct nlattr *attr)
> >  {
> > +	struct nlattr **tb;
> >  	int ret = 0;
> >  
> > -	if (likely(l4proto->ctnl_timeout.nlattr_to_obj)) {
> > -		struct nlattr *tb[l4proto->ctnl_timeout.nlattr_max+1];
> > +	if (!l4proto->ctnl_timeout.nlattr_to_obj)
> > +		return 0;
> 
> Why not
> 	if unlikely(!...)

This is control plane code - not packet path - I think we should just
let the compiler decide on this one, not really need to provide an
explicit hint here.

Re: [PATCH] netfilter: cttimeout: remove VLA usage

[Joe Perches]

On Tue, 2018-03-13 at 15:59 +0100, Pablo Neira Ayuso wrote:
> On Mon, Mar 12, 2018 at 04:58:38PM -0700, Joe Perches wrote:
> > On Mon, 2018-03-12 at 18:14 -0500, Gustavo A. R. Silva wrote:
> > > In preparation to enabling -Wvla, remove VLA and replace it
> > > with dynamic memory allocation.
> > > 
> > > From a security viewpoint, the use of Variable Length Arrays can be
> > > a vector for stack overflow attacks. Also, in general, as the code
> > > evolves it is easy to lose track of how big a VLA can get. Thus, we
> > > can end up having segfaults that are hard to debug.
> > > 
> > > Also, fixed as part of the directive to remove all VLAs from
> > 
> > []
> > > diff --git a/net/netfilter/nfnetlink_cttimeout.c b/net/netfilter/nfnetlink_cttimeout.c
> > 
> > []
> > > @@ -51,19 +51,27 @@ ctnl_timeout_parse_policy(void *timeouts,
> > >  			  const struct nf_conntrack_l4proto *l4proto,
> > >  			  struct net *net, const struct nlattr *attr)
> > >  {
> > > +	struct nlattr **tb;
> > >  	int ret = 0;
> > >  
> > > -	if (likely(l4proto->ctnl_timeout.nlattr_to_obj)) {
> > > -		struct nlattr *tb[l4proto->ctnl_timeout.nlattr_max+1];
> > > +	if (!l4proto->ctnl_timeout.nlattr_to_obj)
> > > +		return 0;
> > 
> > Why not
> > 	if unlikely(!...)
> 
> This is control plane code - not packet path - I think we should just
> let the compiler decide on this one, not really need to provide an
> explicit hint here.

I don't have an issue with that, but it should probably be
mentioned in the changelog as it's unrelated to VLA removal.

Re: [PATCH] netfilter: cttimeout: remove VLA usage

[Pablo Neira Ayuso]

On Mon, Mar 12, 2018 at 06:14:42PM -0500, Gustavo A. R. Silva wrote:
> In preparation to enabling -Wvla, remove VLA and replace it
> with dynamic memory allocation.
> 
> From a security viewpoint, the use of Variable Length Arrays can be
> a vector for stack overflow attacks. Also, in general, as the code
> evolves it is easy to lose track of how big a VLA can get. Thus, we
> can end up having segfaults that are hard to debug.
> 
> Also, fixed as part of the directive to remove all VLAs from
> the kernel: https://lkml.org/lkml/2018/3/7/621

Applied, thanks.