Linux Ethernet Bridging - Jun 2013 - [Bridge] [patch] netfilter: prevent harmless integer overflow

This overflow is harmless because a few lines later we check:

	if (num_counters != t->private->nentries) {

But it still upsets the static checkers.

Signed-off-by: Dan Carpenter <dan.carpenter at oracle.com>

diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
index 3d110c4..141350e 100644
--- a/net/bridge/netfilter/ebtables.c
+++ b/net/bridge/netfilter/ebtables.c
@@ -1278,6 +1278,8 @@ static int do_update_counters(struct net *net, const char
*name,
 
 	if (num_counters == 0)
 		return -EINVAL;
+	if (num_counters > INT_MAX / sizeof(*tmp))
+		return -ENOMEM;
 
 	tmp = vmalloc(num_counters * sizeof(*tmp));
 	if (!tmp)

On Tue, Jun 18, 2013 at 10:46:03AM +0300, Dan Carpenter
wrote:
> This overflow is harmless because a few lines later we check:
> 
> 	if (num_counters != t->private->nentries) {
> 
> But it still upsets the static checkers.
> 
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> 
> diff --git a/net/bridge/netfilter/ebtables.c
b/net/bridge/netfilter/ebtables.c
> index 3d110c4..141350e 100644
> --- a/net/bridge/netfilter/ebtables.c
> +++ b/net/bridge/netfilter/ebtables.c
> @@ -1278,6 +1278,8 @@ static int do_update_counters(struct net *net, const
char *name,
>  
>  	if (num_counters == 0)
>  		return -EINVAL;
> +	if (num_counters > INT_MAX / sizeof(*tmp))
> +		return -ENOMEM;
This is artificially limiting to INT_MAX / sizeof(struct counters).
Before this patch, the limit is UINT_MAX / sizeof(struct counters). I
think it''s very unlikely to hit such a limit though, but as you
mentioned we cover the overflow already. Adding it to calm down a
static checker sound a bit too much for me.

On Tue, Jun 18, 2013 at 10:46:03AM +0300, Dan Carpenter
wrote:
> This overflow is harmless because a few lines later we check:
> 
> 	if (num_counters != t->private->nentries) {
> 
> But it still upsets the static checkers.
> 
> Signed-off-by: Dan Carpenter <dan.carpenter at oracle.com>
> 
> diff --git a/net/bridge/netfilter/ebtables.c
b/net/bridge/netfilter/ebtables.c
> index 3d110c4..141350e 100644
> --- a/net/bridge/netfilter/ebtables.c
> +++ b/net/bridge/netfilter/ebtables.c
> @@ -1278,6 +1278,8 @@ static int do_update_counters(struct net *net, const
char *name,
>  
>  	if (num_counters == 0)
>  		return -EINVAL;
> +	if (num_counters > INT_MAX / sizeof(*tmp))
> +		return -ENOMEM;
This is artificially limiting to INT_MAX / sizeof(struct counters).
Before this patch, the limit is UINT_MAX / sizeof(struct counters). I
think it's very unlikely to hit such a limit though, but as you
mentioned we cover the overflow already. Adding it to calm down a
static checker sound a bit too much for me.

On Thu, Jun 20, 2013 at 12:23:52PM +0200, Pablo Neira Ayuso
wrote:
> On Tue, Jun 18, 2013 at 10:46:03AM +0300, Dan Carpenter wrote:
> > This overflow is harmless because a few lines later we check:
> > 
> > 	if (num_counters != t->private->nentries) {
> > 
> > But it still upsets the static checkers.
> > 
> > Signed-off-by: Dan Carpenter <dan.carpenter at oracle.com>
> > 
> > diff --git a/net/bridge/netfilter/ebtables.c
b/net/bridge/netfilter/ebtables.c
> > index 3d110c4..141350e 100644
> > --- a/net/bridge/netfilter/ebtables.c
> > +++ b/net/bridge/netfilter/ebtables.c
> > @@ -1278,6 +1278,8 @@ static int do_update_counters(struct net *net,
const char *name,
> >  
> >  	if (num_counters == 0)
> >  		return -EINVAL;
> > +	if (num_counters > INT_MAX / sizeof(*tmp))
> > +		return -ENOMEM;
> 
> This is artificially limiting to INT_MAX / sizeof(struct counters).
> Before this patch, the limit is UINT_MAX / sizeof(struct counters). I
> think it's very unlikely to hit such a limit though, but as you
> mentioned we cover the overflow already.  Adding it to calm down a
> static checker sound a bit too much for me.
I think we may be talking about different things?

"num_counters" comes from the user in update_counters() and we can
definitely overflow.  I just copied the checks from do_replace() so
that's why it uses INT_MAX instead of UINT_MAX.

Like I said, the overflow is not harmful because later in the
function we check "(num_counters != t->private->nentries)" and
return an error before using "tmp".  So I don't feel strongly
about
this patch either way.

regards,
dan carpenter