Re: [PATCH] tpm: add buffer access validation in tpm2_get_pcr_allocation()

[Nayna <nayna@linux.vnet.ibm.com>]

On 01/29/2017 08:10 PM, Jarkko Sakkinen wrote:
> On Fri, Jan 27, 2017 at 10:25:49AM -0500, Nayna Jain wrote:
>> This patch add validation in tpm2_get_pcr_allocation to avoid
>> access beyond response buffer length.
>>
>> Suggested-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
>> Signed-off-by: Nayna Jain <nayna@linux.vnet.ibm.com>
>
> This validation looks broken to me.
>
>> ---
>>   drivers/char/tpm/tpm2-cmd.c | 28 +++++++++++++++++++++++-----
>>   1 file changed, 23 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
>> index 4aad84c..02c1ea7 100644
>> --- a/drivers/char/tpm/tpm2-cmd.c
>> +++ b/drivers/char/tpm/tpm2-cmd.c
>> @@ -1008,9 +1008,13 @@ static ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
>>   	struct tpm2_pcr_selection pcr_selection;
>>   	struct tpm_buf buf;
>>   	void *marker;
>> -	unsigned int count = 0;
>> +	void *end;
>> +	void *pcr_select_offset;
>> +	unsigned int count;
>> +	u32 sizeof_pcr_selection;
>> +	u32 resp_len;
>
> Very cosmetic but we almos almost universally use the acronym 'rsp' in
> the TPM driver.

Sure will update.

>
>>   	int rc;
>> -	int i;
>> +	int i = 0;
>
> Why do you need to initialize it?

Because in out: count is replaced with i.
And it is replaced because  now for loop can break even before reaching 
count, because of new buffer checks.
>
>>
>>   	rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_GET_CAPABILITY);
>>   	if (rc)
>> @@ -1034,15 +1038,29 @@ static ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
>>   	}
>>
>>   	marker = &buf.data[TPM_HEADER_SIZE + 9];
>> +
>> +	resp_len = be32_to_cpup((__be32 *)&buf.data[2]);
>> +	end = &buf.data[resp_len];
>
> What if the response contains larger length than the buffer size?

Isn't this check need to be done in tpm_transmit_cmd for all responses ?
Though, it seems it is not done there as well.

And to understand what do we expect max buffer length. PAGE_SIZE or 
TPM_BUFSIZE ?

>
>> +
>>   	for (i = 0; i < count; i++) {
>> +		pcr_select_offset = marker +
>> +			offsetof(struct tpm2_pcr_selection, size_of_select);
>> +		if (pcr_select_offset >= end) {
>> +			rc = -EFAULT;
>> +			break;
>> +		}
>> +
>>   		memcpy(&pcr_selection, marker, sizeof(pcr_selection));
>>   		chip->active_banks[i] = be16_to_cpu(pcr_selection.hash_alg);
>> -		marker = marker + sizeof(struct tpm2_pcr_selection);
>> +		sizeof_pcr_selection = sizeof(pcr_selection.hash_alg) +
>> +			sizeof(pcr_selection.size_of_select) +
>> +			sizeof(u8) * pcr_selection.size_of_select;
>
> Remove "sizeof(u8) * ".

Sure.
>
>> +		marker = marker + sizeof_pcr_selection;
>>   	}
>>
>>   out:
>> -	if (count < ARRAY_SIZE(chip->active_banks))
>> -		chip->active_banks[count] = TPM2_ALG_ERROR;
>> +	if (i < ARRAY_SIZE(chip->active_banks))
>> +		chip->active_banks[i] = TPM2_ALG_ERROR;
>>
>>   	tpm_buf_destroy(&buf);
>>
>> --
>> 2.5.0
>>
>
> I'm sorry but this commit is changing too much. You need to redo the
> whole commit and resend the patch set with these fixes. You can keep
> Reviewed-by and Tested-by in 1/2 but have to remove them from 2/2.

Sure, will do.

Thanks & Regards,
    - Nayna


>
> /Jarkko
>