Re: [PATCH] nfc: st-nci: array index overflow in st_nci_se_get_bwi()

[Alexander Duyck]

On Mon, Dec 19, 2022 at 1:06 AM Krzysztof Kozlowski
<krzysztof.kozlowski@xxxxxxxxxx> wrote:
>
> On 14/12/2022 19:35, Alexander H Duyck wrote:
> > On Tue, 2022-12-13 at 09:12 -0500, Aleksandr Burakov wrote:
> >> Index of info->se_info.atr can be overflow due to unchecked increment
> >> in the loop "for". The patch checks the value of current array index
> >> and doesn't permit increment in case of the index is equal to
> >> ST_NCI_ESE_MAX_LENGTH - 1.
> >>
> >> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> >>
> >> Fixes: ed06aeefdac3 ("nfc: st-nci: Rename st21nfcb to st-nci")
> >> Signed-off-by: Aleksandr Burakov <a.burakov@xxxxxxxxxxxx>
> >> ---
> >>  drivers/nfc/st-nci/se.c | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/nfc/st-nci/se.c b/drivers/nfc/st-nci/se.c
> >> index ec87dd21e054..ff8ac1784880 100644
> >> --- a/drivers/nfc/st-nci/se.c
> >> +++ b/drivers/nfc/st-nci/se.c
> >> @@ -119,10 +119,11 @@ static u8 st_nci_se_get_bwi(struct nci_dev *ndev)
> >>      /* Bits 8 to 5 of the first TB for T=1 encode BWI from zero to nine */
> >>      for (i = 1; i < ST_NCI_ESE_MAX_LENGTH; i++) {
> >>              td = ST_NCI_ATR_GET_Y_FROM_TD(info->se_info.atr[i]);
> >> -            if (ST_NCI_ATR_TA_PRESENT(td))
> >> +            if (ST_NCI_ATR_TA_PRESENT(td) && i < ST_NCI_ESE_MAX_LENGTH - 1)
> >>                      i++;
> >>              if (ST_NCI_ATR_TB_PRESENT(td)) {
> >> -                    i++;
> >> +                    if (i < ST_NCI_ESE_MAX_LENGTH - 1)
> >> +                            i++;
> >>                      return info->se_info.atr[i] >> 4;
> >>              }
> >>      }
> >
> > Rather than adding 2 checks you could do this all with one check.
> > Basically you would just need to replace:
> >   if (ST_NCI_ATR_TB_PRESENT(td)) {
> >       i++;
> >
> > with:
> >   if (ST_NCI_ATR_TB_PRESENT(td) && ++i < ST_NCI_ESE_MAX_LENGTH)
> >
> > Basically it is fine to increment "i" as long as it isn't being used as
> > an index so just restricting the last access so that we don't
> > dereference using it as an index should be enough.
>
> These are different checks - TA and TB. By skipping TA, your code is not
> equivalent. Was it intended?

Sorry, I wasn't talking about combining the TA and TB checks. I was
talking about combining the TB check and the bounds check so that you
didn't return and se_info_atr for a value that may not have actually
aligned due to the fact you had overflowed. Specifically, is skipping
the i++ the correct response to going out of bounds? I'm wondering if
you should be returning the default instead in the case of overflow?

The TA check could be modified so that it checks for "++i =
ST_NCI_ESE_MAX_LENGTH" and if that is true break rather than continue
in the loop.