linux-kernel - Re: [PATCH v3 2/2] Make num

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Date:   Thu, 28 Sep 2023 19:55:09 -0700
From:   Joao Moreira <joao@...rdrivepizza.com>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kadlec@...filter.org, fw@...len.de, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        rkannoth@...vell.com, wojciech.drewek@...el.com,
        steen.hegenlund@...rohip.com, keescook@...omium.org,
        Joao Moreira <joao.moreira@...el.com>
Subject: Re: [PATCH v3 2/2] Make num_actions unsigned

On 2023-09-28 06:43, Pablo Neira Ayuso wrote:
> On Wed, Sep 27, 2023 at 09:47:15AM -0700, joao@...rdrivepizza.com 
> wrote:
>> From: Joao Moreira <joao.moreira@...el.com>
>> 
>> Currently, in nft_flow_rule_create function, num_actions is a signed
>> integer. Yet, it is processed within a loop which increments its
>> value. To prevent an overflow from occurring, make it unsigned and
>> also check if it reaches 256 when being incremented.
>> 
>> Accordingly to discussions around v2, 256 actions are more than enough
>> for the frontend actions.
>> 
>> After checking with maintainers, it was mentioned that front-end will
>> cap the num_actions value and that it is not possible to reach such
>> condition for an overflow. Yet, for correctness, it is still better to
>> fix this.
>> 
>> This issue was observed by the commit author while reviewing a 
>> write-up
>> regarding a CVE within the same subsystem [1].
>> 
>> 1 - https://nickgregory.me/post/2022/03/12/cve-2022-25636/
> 
> Yes, but this is not related to the netfilter subsystem itself, this
> harderning is good to have for the flow offload infrastructure in
> general.

Right, I'll try to look up where this would fit in then. I'm not an 
expert in the subsystem at all, so should take a minute or two for me to 
get to it and send a v4.

> 
>> Signed-off-by: Joao Moreira <joao.moreira@...el.com>
>> ---
>>  net/netfilter/nf_tables_offload.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>> 
>> diff --git a/net/netfilter/nf_tables_offload.c 
>> b/net/netfilter/nf_tables_offload.c
>> index 12ab78fa5d84..9a86db1f0e07 100644
>> --- a/net/netfilter/nf_tables_offload.c
>> +++ b/net/netfilter/nf_tables_offload.c
>> @@ -90,7 +90,8 @@ struct nft_flow_rule *nft_flow_rule_create(struct 
>> net *net,
>>  {
>>  	struct nft_offload_ctx *ctx;
>>  	struct nft_flow_rule *flow;
>> -	int num_actions = 0, err;
>> +	unsigned int num_actions = 0;
>> +	int err;
> 
> reverse xmas tree.

ack.

> 
>>  	struct nft_expr *expr;
>> 
>>  	expr = nft_expr_first(rule);
>> @@ -99,6 +100,10 @@ struct nft_flow_rule *nft_flow_rule_create(struct 
>> net *net,
>>  		    expr->ops->offload_action(expr))
>>  			num_actions++;
>> 
>> +		/* 2^8 is enough for frontend actions, avoid overflow */
>> +		if (num_actions == 256)
> 
> This cap is not specific of nf_tables, it should apply to all other
> subsystems. This is the wrong spot.

Any pointers regarding where I should look at?

> 
> Moreover, please, add a definition for this, no hardcoded values.

Ack.

> 
>> +			return ERR_PTR(-ENOMEM);
> 
> Better E2BIG or similar, otherwise this propagates to userspace as
> ENOMEM.

Ack.

> 
>> +
>>  		expr = nft_expr_next(expr);
>>  	}
>> 
>> --
>> 2.42.0
>>