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Subject: Re: [PATCH] i2c: core-smbus: fix a potential uninitialization bug
To:     Wenwen Wang <wang6495@umn.edu>
Cc:     Kangjie Lu <kjlu@umn.edu>, Wolfram Sang <wsa@the-dreams.de>,
        "open list:I2C SUBSYSTEM" <linux-i2c@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
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From:   Peter Rosin <peda@axentia.se>
Organization: Axentia Technologies AB
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Date:   Fri, 4 May 2018 07:04:41 +0200
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On 2018-05-04 06:08, Wenwen Wang wrote:
> On Thu, May 3, 2018 at 3:34 PM, Peter Rosin <peda@axentia.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@umn.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;
>>>
>>