Re: [PATCH v2] nfc: pn533: Add poll mod list filling check

[Paolo Abeni]

On 8/27/24 10:48, Aleksandr Mishin wrote:
> In case of im_protocols value is 1 and tm_protocols value is 0 this
> combination successfully passes the check
> 'if (!im_protocols && !tm_protocols)' in the nfc_start_poll().
> But then after pn533_poll_create_mod_list() call in pn533_start_poll()
> poll mod list will remain empty and dev->poll_mod_count will remain 0
> which lead to division by zero.
>
> Normally no im protocol has value 1 in the mask, so this combination is
> not expected by driver. But these protocol values actually come from
> userspace via Netlink interface (NFC_CMD_START_POLL operation). So a
> broken or malicious program may pass a message containing a "bad"
> combination of protocol parameter values so that dev->poll_mod_count
> is not incremented inside pn533_poll_create_mod_list(), thus leading
> to division by zero.
> Call trace looks like:
> nfc_genl_start_poll()
>    nfc_start_poll()
>      ->start_poll()
>      pn533_start_poll()
>
> Add poll mod list filling check.
>
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>
> Fixes: dfccd0f58044 ("NFC: pn533: Add some polling entropy")
> Signed-off-by: Aleksandr Mishin <amishin@xxxxxxxxxx>

The issue looks real to me and the proposed fix the correct one, but 
waiting a little more for Krzysztof feedback, as he expressed concerns 
on v1.

Thanks,

Paolo