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

[Paolo Abeni]

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@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.
>
> 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@xxxxxxxxxx>
>
> 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