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

[Krzysztof Kozlowski <krzk@kernel.org>]

On 29/08/2024 11:34, Paolo Abeni wrote:
> 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.

Yeah, the patch I already acked, can go in. I am just thinking whether
we need a follow-up for core NFC code.

Best regards,
Krzysztof