Re: [PATCH] i2c: core-smbus: fix a potential uninitialization bug

[Wenwen Wang]

On Fri, May 4, 2018 at 1:49 AM, Peter Rosin <peda@xxxxxxxxxx> wrote:
> On 2018-05-04 07:28, Wenwen Wang wrote:
>> On Fri, May 4, 2018 at 12:04 AM, Peter Rosin <peda@xxxxxxxxxx> wrote:
>>> On 2018-05-04 06:08, Wenwen Wang wrote:
>>>> On Thu, May 3, 2018 at 3:34 PM, Peter Rosin <peda@xxxxxxxxxx> wrote:
>>>>> On 2018-05-03 00:36, Wenwen Wang wrote:
>>>>>> In i2c_smbus_xfer_emulated(), there are two buffers: msgbuf0 and msgbuf1,
>>>>>> which are used to save a series of messages, as mentioned in the comment.
>>>>>> According to the value of the variable "size", msgbuf0 is initialized to
>>>>>> various values. In contrast, msgbuf1 is left uninitialized until the
>>>>>> function i2c_transfer() is invoked. However, mgsbuf1 is not always
>>>>>> initialized on all possible execution paths (implementation) of
>>>>>> i2c_transfer(). Thus, it is possible that mgsbuf1 may still not be
>>>>>
>>>>> double negation here
>>>>>
>>>>>> uninitialized even after the invocation of the function i2c_transfer(). In
>>>>>> the following execution, the uninitialized msgbuf1 will be used, such as
>>>>>> for security checks. Since uninitialized values can be random and
>>>>>> arbitrary, this will cause undefined behaviors or even check bypass. For
>>>>>> example, it is expected that if the value of "size" is
>>>>>> I2C_SMBUS_BLOCK_PROC_CALL, the value of data->block[0] should not be larger
>>>>>> than I2C_SMBUS_BLOCK_MAX. But, at the end of i2c_smbus_xfer_emulated(), the
>>>>>> value read from msgbuf1 is assigned to data->block[0], which can
>>>>>> potentially lead to invalid block write size, as demonstrated in the error
>>>>>> message.
>>>>>>
>>>>>> This patch simply initializes the buffer msgbuf1 with 0 to avoid undefined
>>>>>> behaviors or security issues.
>>>>>>
>>>>>> Signed-off-by: Wenwen Wang <wang6495@xxxxxxx>
>>>>>> ---
>>>>>>  drivers/i2c/i2c-core-smbus.c | 2 +-
>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/i2c/i2c-core-smbus.c b/drivers/i2c/i2c-core-smbus.c
>>>>>> index b5aec33..0fcca75 100644
>>>>>> --- a/drivers/i2c/i2c-core-smbus.c
>>>>>> +++ b/drivers/i2c/i2c-core-smbus.c
>>>>>> @@ -324,7 +324,7 @@ static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
>>>>>>        * somewhat simpler.
>>>>>>        */
>>>>>>       unsigned char msgbuf0[I2C_SMBUS_BLOCK_MAX+3];
>>>>>> -     unsigned char msgbuf1[I2C_SMBUS_BLOCK_MAX+2];
>>>>>> +     unsigned char msgbuf1[I2C_SMBUS_BLOCK_MAX+2] = {0};
>>>>>
>>>>> I think this will result in the whole of msgbuf1 being filled with zeroes.
>>>>> It might be cheaper to do this with code proper rather than with an
>>>>> initializer?
>>>>
>>>> Thanks for your comment, Peter!  How about using a memset() only when
>>>> i2c_smbus_xfer_emulated() emulates reading commands, since msgbuf1 is
>>>> used only in that case?
>>>
>>> I was thinking that an assignment of
>>>
>>>         msgbuf1[0] = 0;
>>>
>>> would be enough in the I2C_SMBUS_BLOCK_DATA and I2C_SMBUS_BLOCK_PROC_CALL
>>> cases before the i2c_transfer call. However, this will only kick in if
>>> the call to kzalloc fails (and it most likely will not) in the call to the
>>> i2c_smbus_try_get_dmabuf helper. So, this thing that you are trying to fix
>>> seems like a non-issue to me.
>>>
>>> However, while looking I think the bigger problem with that function is that
>>> it considers all non-negative return values from i2c_transfer as good<tm>.
>>> IMHO, it should barf on any return values <> num. Or at the very least
>>> describe why a partial result is considered OK...
>>>
>>> Cheers,
>>> Peter
>>>
>>>>>
>>>>> Cheers,
>>>>> Peter
>>>>>
>>>>>>       int num = read_write == I2C_SMBUS_READ ? 2 : 1;
>>>>>>       int i;
>>>>>>       u8 partial_pec = 0;
>>>>>>
>>>>>
>>>
>>
>> Yes, it is a big issue if the return value from i2c_transfer() is not
>> equal to num. I can add a check like this:
>>
>> if (status != num)
>>   return -EINVAL;
>>
>
> Right, but make sure to add it *after* the existing "if (status < 0)"
> check as we want to preserve any existing error. Also, -EIO is perhaps
> more appropriate than -EINVAL which seems wrong for what is probably
> a runtime incident.
>

Sure, I will place it after the existing check and replace -EINVAL with -EIO.

>> Also, I wonder why msgbuf1 is necessary if it is replaced by kzalloc
>> in i2c_smbus_try_get_dmabuf()?
>
> It is not always replaced. The stack buffer is probably retained as
> the default mode of operation (and fallback) because kzalloc is
> expensive and because kzalloc might fail?
>

That means the stack buffer is probably used if kzalloc is failed.
Actually, the kzalloc failure would be possible if a user-space
process maliciously causes the kernel to consume a large chunk of
memory. In that case, the user can potentially exploit this
problematic code. So it may be better to initialize the stack buffer.

Thanks,
Wenwen

> Cheers,
> Peter
>
>> Thanks,
>> Wenwen
>>
>