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

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Message-ID: <4390a69e-a297-313d-044d-abf846eff1d1@axentia.se>
Date:   Thu, 3 May 2018 22:34:06 +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 2018-05-03 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?

Cheers,
Peter

>  	int num = read_write == I2C_SMBUS_READ ? 2 : 1;
>  	int i;
>  	u8 partial_pec = 0;
>