Re: [PATCH v2 0/2] Prevent potential write out of bounds

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, Sep 27, 2023 at 10:25:03AM +0200, Pablo Neira Ayuso wrote:
> On Tue, Sep 26, 2023 at 07:02:19PM -0700, joao@overdrivepizza.com wrote:
> > From: Joao Moreira <joao.moreira@intel.com>
> > 
> > The function flow_rule_alloc in net/core/flow_offload.c [2] gets an
> > unsigned int num_actions (line 10) and later traverses the actions in
> > the rule (line 24) setting hw.stats to FLOW_ACTION_HW_STATS_DONT_CARE.
> > 
> > Within the same file, the loop in the line 24 compares a signed int
> > (i) to an unsigned int (num_actions), and then uses i as an array
> > index. If an integer overflow happens, then the array within the loop
> > is wrongly indexed, causing a write out of bounds.
> > 
> > After checking with maintainers, it seems that the front-end caps the
> > maximum value of num_action, thus it is not possible to reach the given
> > write out of bounds, yet, still, to prevent disasters it is better to
> > fix the signedness here.
> > 
> > Similarly, also it is also good to ensure that an overflow won't happen
> > in net/netfilter/nf_tables_offload.c's function nft_flow_rule_create by
> > making the variable unsigned and ensuring that it returns an error if
> > its value reaches UINT_MAX.
> > 
> > This issue was observed by the commit author while reviewing a write-up
> > regarding a CVE within the same subsystem [1].
> 
> I keep spinning around this, this is not really an issue.
> 
> No frontend uses this amount of actions.
> 
> Probably cap this to uint16_t because 2^16 actions is more than
> sufficient by now.

Actually, even 2^8 actions is more than enough by now.