[PATCH] netfilter: nf_tables: prevent shift wrap in nft_chain_parse_hook()

[PATCH] netfilter: nf_tables: prevent shift wrap in nft_chain_parse_hook()

[Dan Carpenter]

I believe that "hook->num" can be up to UINT_MAX so the shift has to
be unsigned or shifting more than 31 is undefined.  It's possible that
this leads to array overflow in nf_tables_addchain():

	ops->hook	= hook.type->hooks[ops->hooknum];

Fixes: fe19c04ca137 ("netfilter: nf_tables: remove nhooks field from struct nft_af_info")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
 net/netfilter/nf_tables_api.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index ef7772e976cc..b6e48fe7c776 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -1545,7 +1545,7 @@ static int nft_chain_parse_hook(struct net *net,
 		if (IS_ERR(type))
 			return PTR_ERR(type);
 	}
-	if (!(type->hook_mask & (1 << hook->num)))
+	if (!(type->hook_mask & (1U << hook->num)))
 		return -EOPNOTSUPP;
 
 	if (type->type == NFT_CHAIN_T_NAT &&
-- 
2.17.1

Re: [PATCH] netfilter: nf_tables: prevent shift wrap in nft_chain_parse_hook()

[Dan Carpenter]

On Tue, Apr 02, 2019 at 04:30:38PM +0300, Dan Carpenter wrote:
> I believe that "hook->num" can be up to UINT_MAX so the shift has to
> be unsigned or shifting more than 31 is undefined.  It's possible that
> this leads to array overflow in nf_tables_addchain():
> 
> 	ops->hook	= hook.type->hooks[ops->hooknum];
> 
> Fixes: fe19c04ca137 ("netfilter: nf_tables: remove nhooks field from struct nft_af_info")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> ---
>  net/netfilter/nf_tables_api.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
> index ef7772e976cc..b6e48fe7c776 100644
> --- a/net/netfilter/nf_tables_api.c
> +++ b/net/netfilter/nf_tables_api.c
> @@ -1545,7 +1545,7 @@ static int nft_chain_parse_hook(struct net *net,
>  		if (IS_ERR(type))
>  			return PTR_ERR(type);
>  	}
> -	if (!(type->hook_mask & (1 << hook->num)))
> +	if (!(type->hook_mask & (1U << hook->num)))

Ugh...  No this doesn't work.  It's still undefined behavoir even if we
make it unsigned.  It has to be:

	if (hook->num > 31 || !(type->hook_mask & (1 << hook->num)))

which seems like a slightly ugly magic number but it fixes the buffer
overflow.

regards,
dan carpenter

Re: [PATCH] netfilter: nf_tables: prevent shift wrap in nft_chain_parse_hook()

[Florian Westphal]

Dan Carpenter <dan.carpenter@oracle.com> wrote:
> 	if (hook->num > 31 || !(type->hook_mask & (1 << hook->num)))
> 
> which seems like a slightly ugly magic number but it fixes the buffer
> overflow.

You can use NF_MAX_HOOKS instead of 31.

[PATCH v2] netfilter: nf_tables: prevent shift wrap in nft_chain_parse_hook()

[Dan Carpenter]

I believe that "hook->num" can be up to UINT_MAX.  Shifting more than
31 bits would is undefined in C but in practice it would lead to shift
wrapping.  That would lead to an array overflow in nf_tables_addchain():

	ops->hook       = hook.type->hooks[ops->hooknum];

Fixes: fe19c04ca137 ("netfilter: nf_tables: remove nhooks field from struct nft_af_info")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
v2:  In my original fix I thought making shifts unsigned was defined in
C but it's not.  My original patch didn't fix or change runtime at all.

Thanks to Florian Westphal for his help on this.

 net/netfilter/nf_tables_api.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index ef7772e976cc..1606eaa5ae0d 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -1545,7 +1545,7 @@ static int nft_chain_parse_hook(struct net *net,
 		if (IS_ERR(type))
 			return PTR_ERR(type);
 	}
-	if (!(type->hook_mask & (1 << hook->num)))
+	if (hook->num > NF_MAX_HOOKS || !(type->hook_mask & (1 << hook->num)))
 		return -EOPNOTSUPP;
 
 	if (type->type == NFT_CHAIN_T_NAT &&
-- 
2.17.1

Re: [PATCH v2] netfilter: nf_tables: prevent shift wrap in nft_chain_parse_hook()

[Pablo Neira Ayuso]

On Sat, Apr 06, 2019 at 08:26:52AM +0300, Dan Carpenter wrote:
> I believe that "hook->num" can be up to UINT_MAX.  Shifting more than
> 31 bits would is undefined in C but in practice it would lead to shift
> wrapping.  That would lead to an array overflow in nf_tables_addchain():
> 
> 	ops->hook       = hook.type->hooks[ops->hooknum];

Applied, thanks.