Re: i2c: core-smbus: prevent stack corruption on read I2C_BLOCK_DATA

[Wolfram Sang <wsa@the-dreams.de>]

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On Wed, Nov 15, 2017 at 12:54:09PM -0700, Compostella, Jeremy wrote:
> On a I2C_SMBUS_I2C_BLOCK_DATA read request, if data->block[0] is
> greater than I2C_SMBUS_BLOCK_MAX + 1, the underlying I2C driver writes

greater or equal?

> data out of the msgbuf1 boundary.
> 
> It is possible from a user application to run into that issue by call
> the I2C_SMBUS ioctl with data.block[0] greater than
> I2C_SMBUS_BLOCK_MAX + 1.

ditto?

> 
> Call Trace:
>  [<ffffffff8139f695>] dump_stack+0x67/0x92
>  [<ffffffff811802a4>] panic+0xc5/0x1eb
>  [<ffffffff810ecb5f>] ? vprintk_default+0x1f/0x30
>  [<ffffffff817456d3>] ? i2cdev_ioctl_smbus+0x303/0x320
>  [<ffffffff8109a68b>] __stack_chk_fail+0x1b/0x20
>  [<ffffffff817456d3>] i2cdev_ioctl_smbus+0x303/0x320
>  [<ffffffff81745aed>] i2cdev_ioctl+0x4d/0x1e0
>  [<ffffffff811f761a>] do_vfs_ioctl+0x2ba/0x490
>  [<ffffffff81336e43>] ? security_file_ioctl+0x43/0x60
>  [<ffffffff811f7869>] SyS_ioctl+0x79/0x90
>  [<ffffffff81a22e97>] entry_SYSCALL_64_fastpath+0x12/0x6a
> 
> Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
> ---
>  drivers/i2c/i2c-core-smbus.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/i2c/i2c-core-smbus.c b/drivers/i2c/i2c-core-smbus.c
> index 10f00a8..f0be621 100644
> --- a/drivers/i2c/i2c-core-smbus.c
> +++ b/drivers/i2c/i2c-core-smbus.c
> @@ -398,6 +398,12 @@ static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
>  	case I2C_SMBUS_I2C_BLOCK_DATA:
>  		if (read_write == I2C_SMBUS_READ) {
>  			msg[1].len = data->block[0];
> +			if (msg[1].len > I2C_SMBUS_BLOCK_MAX) {
> +				dev_err(&adapter->dev,
> +					"Invalid block read size %d\n",
> +					data->block[0]);
> +				return -EINVAL;
> +			}

Basically good, yet to make the code easier to read I'd suggest
something like this? Untested, wanted to hear your opinion first.

 drivers/i2c/i2c-core-smbus.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/i2c/i2c-core-smbus.c b/drivers/i2c/i2c-core-smbus.c
index 4bb9927afd0106..5fc5ddd965434f 100644
--- a/drivers/i2c/i2c-core-smbus.c
+++ b/drivers/i2c/i2c-core-smbus.c
@@ -397,16 +397,16 @@ static s32 i2c_smbus_xfer_emulated(struct i2c_adapter *adapter, u16 addr,
 				   the underlying bus driver */
 		break;
 	case I2C_SMBUS_I2C_BLOCK_DATA:
+		if (data->block[0] > I2C_SMBUS_BLOCK_MAX) {
+			dev_err(&adapter->dev, "Invalid block size %d\n",
+				data->block[0]);
+			return -EINVAL;
+		}
+
 		if (read_write == I2C_SMBUS_READ) {
 			msg[1].len = data->block[0];
 		} else {
 			msg[0].len = data->block[0] + 1;
-			if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 1) {
-				dev_err(&adapter->dev,
-					"Invalid block write size %d\n",
-					data->block[0]);
-				return -EINVAL;
-			}
 			for (i = 1; i <= data->block[0]; i++)
 				msgbuf0[i] = data->block[i];
 		}

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