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

[Paolo Abeni <pabeni@redhat.com>]

On 8/29/24 11:06, Krzysztof Kozlowski wrote:
> On 29/08/2024 10:26, Paolo Abeni wrote:
>>
>>
>> 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@t-argos.ru>
>>
>> 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.
> 
> There was one month delay between my reply and clarifications from
> Fedor, so original patch is neither in my mailbox nor in my brain.
> 
> 
> Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> 
> However different problem is: shouldn't as well or instead
> nfc_genl_start_poll() validate the attributes received by netlink?
> 
> We just pass them directly to the drivers and several other drivers
> might not expect random stuff there.

FTR, I had a similar thought and skimmed over other nfc drivers. I did 
not see similar issues there.

Additionally I fear that existing user-space could feed to the kernel 
such random stuff and work happily because the kernel is currently 
ignoring it - on other drivers. Such cases will suddenly stop working.

I think we could/should merge the patch as-is, please LMK your thought.

Thanks,

Paolo