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

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

On 01/30/2017 02:50 AM, Jarkko Sakkinen wrote:
> On Sun, Jan 29, 2017 at 10:48:39PM +0530, Nayna wrote:
>>
>>
>> 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 ?
>
> Oops. You are correct it is done there:
>
> if (len != be32_to_cpu(header->length))
> 	return -EFAULT;
>
> So need to do this.

To be sure, means nothing need to be done in this. Right ?

And guess this was the only thing you meant by broken for this patch.

I will do other two smaller changes as I send the whole new patchset.

Thanks & Regards,
   - Nayna

>
> /Jarkko
>
> /Jarkko
>