Date:   Fri, 4 May 2018 20:28:35 -0500
From:   Wenwen Wang <wang6495@....edu>
To:     Peter Rosin <peda@...ntia.se>
Cc:     Kangjie Lu <kjlu@....edu>, Wolfram Sang <wsa@...-dreams.de>,
        "open list:I2C SUBSYSTEM" <linux-i2c@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Wenwen Wang <wang6495@....edu>
Subject: Re: [PATCH] i2c: core-smbus: fix a potential uninitialization bug

On Fri, May 4, 2018 at 10:38 AM, Peter Rosin <peda@...ntia.se> wrote:
> On 2018-05-04 16:59, Wenwen Wang wrote:
>> On Fri, May 4, 2018 at 2:27 AM, Peter Rosin <peda@...ntia.se> wrote:
>>> On 2018-05-04 09:17, Wenwen Wang wrote:
>>>> On Fri, May 4, 2018 at 1:49 AM, Peter Rosin <peda@...ntia.se> wrote:
>>>>> On 2018-05-04 07:28, Wenwen Wang wrote:
>>>>>> On Fri, May 4, 2018 at 12:04 AM, Peter Rosin <peda@...ntia.se> wrote:
>>>>>>> On 2018-05-04 06:08, Wenwen Wang wrote:
>>>>>>>> On Thu, May 3, 2018 at 3:34 PM, Peter Rosin <peda@...ntia.se> 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@....edu>
>>>>>>>>>> ---
>>>>>>>>>>  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.
>>>
>>> Yes, but I see little reason to initialize more than the first byte.
>>>
>>> You hinted in the commit message that there were execution paths (or
>>> implementations) where the second buffer wasn't initialized. Can you
>>> give an example where this matters when the more extensive check on
>>> the i2c_transfer return value is in place? That seems like a bugs
>>> that should *also* be fixed in the affected i2c bus drivers...
>>
>> One possible execution path is as follows:
>>
>> i2c_transfer -> __i2c_transfer -> pca_xfer (which is one of the
>> master_xfer handlers)
>>
>> In pca_xfer(), it reads the status of the i2c_adapter and then
>> performs different actions according to different statuses.
>>
>> It seems probable that the buffer is not filled with the wanted data
>> if the status is not as expected.
>
> Ah, so you're talking about hardware malfunction without any actual
> real-life incident. In other words, pure speculation. I'm sure the
> kernel is full of problems if every potential HW misbehavior is
> considered, and I'm not so sure this particular problem is going
> to matter all that much...

Thanks for your comments, Peter! I will submit a new patch :)

Wenwen

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