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

[Krzysztof Kozlowski]

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?

Best regards,
Krzysztof