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

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Date:   Sat, 5 May 2018 12:28:53 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Wenwen Wang <wang6495@....edu>
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>
Subject: Re: [PATCH] i2c: core-smbus: fix a potential uninitialization bug

On 2018-05-05 03:43, 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 be
> uninitialized even after the invocation of the function i2c_transfer(),
> especially when the return value of ic2_transfer() is not checked properly.
> 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 checks the return value of i2c_transfer() and also initializes
> the first byte of msgbuf1 with 0 to avoid undefined behaviors or security
> issues.
> 
> Signed-off-by: Wenwen Wang <wang6495@....edu>
> ---
>  drivers/i2c/i2c-core-smbus.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/i2c/i2c-core-smbus.c b/drivers/i2c/i2c-core-smbus.c
> index b5aec33..e8470d5 100644
> --- a/drivers/i2c/i2c-core-smbus.c
> +++ b/drivers/i2c/i2c-core-smbus.c
> @@ -344,6 +344,7 @@ static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
>  	};
>  
>  	msgbuf0[0] = command;
> +	msgbug1[0] = 0;
>  	switch (size) {
>  	case I2C_SMBUS_QUICK:
>  		msg[0].len = 0;
> @@ -466,6 +467,8 @@ static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
>  	status = i2c_transfer(adapter, msg, num);
>  	if (status < 0)
>  		return status;
> +	if (status != num)
> +		return -EIO;
>  
>  	/* Check PEC if last message is a read */
>  	if (i && (msg[num-1].flags & I2C_M_RD)) {
> 

I think these two hunks should be two separate patches. They address
orthogonal issues...

Cheers,
Peter