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SubjectRe: [PATCH] nfc: st-nci: array index overflow in st_nci_se_get_bwi()
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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@rosalinux.ru>
>> ---
>> 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?

Best regards,
Krzysztof

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