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SubjectRe: [PATCH] i2c: core-smbus: fix a potential uninitialization bug
On Fri, May 4, 2018 at 2:27 AM, Peter Rosin <> wrote:
> On 2018-05-04 09:17, Wenwen Wang wrote:
>> On Fri, May 4, 2018 at 1:49 AM, Peter Rosin <> wrote:
>>> On 2018-05-04 07:28, Wenwen Wang wrote:
>>>> On Fri, May 4, 2018 at 12:04 AM, Peter Rosin <> wrote:
>>>>> On 2018-05-04 06:08, Wenwen Wang wrote:
>>>>>> On Thu, May 3, 2018 at 3:34 PM, Peter Rosin <> 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 <>
>>>>>>>> ---
>>>>>>>> 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.


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