Re: [PATCH v2] i2c: stub: Reject I2C block transfers with invalid length

[Jean Delvare <jdelvare@suse.de>]

Hi Weiming,

On Wed, 15 Apr 2026 01:23:39 +0800, Weiming Shi wrote:
> The I2C_SMBUS_I2C_BLOCK_DATA case in stub_xfer() uses data->block[0]
> as the transfer length. The existing check only clamps it to avoid
> overrunning the chip->words[256] register array, but does not validate
> it against I2C_SMBUS_BLOCK_MAX (32), which is the limit of the union
> i2c_smbus_data.block buffer (34 bytes total). The driver is a
> development/test tool (CONFIG_I2C_STUB=m, not built by default)
> that must be loaded with a chip_addr= parameter.
> 
> A local user with access to /dev/i2c-* can issue an I2C_SMBUS ioctl
> with I2C_SMBUS_I2C_BLOCK_DATA and data->block[0] > 32, causing
> stub_xfer() to read or write past the end of the union
> i2c_smbus_data.block buffer:
> 
>  BUG: KASAN: stack-out-of-bounds in stub_xfer (drivers/i2c/i2c-stub.c:223)
>  Read of size 1 at addr ffff88800abcfd92 by task exploit/81
>  Call Trace:
>   <TASK>
>   stub_xfer (drivers/i2c/i2c-stub.c:223)
>   __i2c_smbus_xfer (drivers/i2c/i2c-core-smbus.c:593)
>   i2c_smbus_xfer (drivers/i2c/i2c-core-smbus.c:536)
>   i2cdev_ioctl_smbus (drivers/i2c/i2c-dev.c:391)
>   i2cdev_ioctl (drivers/i2c/i2c-dev.c:478)
>   __x64_sys_ioctl (fs/ioctl.c:583)
>   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
>   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
>   </TASK>
> 
> The bug exists because i2c-stub implements .smbus_xfer directly,
> bypassing the I2C_SMBUS_BLOCK_MAX validation in
> i2c_smbus_xfer_emulated(). The I2C_SMBUS_BLOCK_DATA case in the same
> function correctly validates against I2C_SMBUS_BLOCK_MAX, but the
> I2C_SMBUS_I2C_BLOCK_DATA case does not.
> 
> Fix by rejecting transfers with data->block[0] == 0 or
> data->block[0] > I2C_SMBUS_BLOCK_MAX with -EINVAL, consistent with
> both the I2C_SMBUS_BLOCK_DATA case in the same function and the
> I2C_SMBUS_I2C_BLOCK_DATA validation in i2c_smbus_xfer_emulated().
> 
> Fixes: 4710317891e4 ("i2c-stub: Implement I2C block support")
> Reported-by: Xiang Mei <xmei5@asu.edu>
> Signed-off-by: Weiming Shi <bestswngs@gmail.com>

Reviewed-by: Jean Delvare <jdelvare@suse.de>

> ---
> v2:
>  - Also reject data->block[0] == 0 (suggested by Jean Delvare).
> 
>  drivers/i2c/i2c-stub.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/i2c/i2c-stub.c b/drivers/i2c/i2c-stub.c
> index fbb0db41b10e1..04314e3ed24c9 100644
> --- a/drivers/i2c/i2c-stub.c
> +++ b/drivers/i2c/i2c-stub.c
> @@ -214,6 +214,11 @@ static s32 stub_xfer(struct i2c_adapter *adap, u16 addr, unsigned short flags,
>  		 * We ignore banks here, because banked chips don't use I2C
>  		 * block transfers
>  		 */
> +		if (data->block[0] == 0 ||
> +		    data->block[0] > I2C_SMBUS_BLOCK_MAX) {
> +			ret = -EINVAL;
> +			break;
> +		}
>  		if (data->block[0] > 256 - command)	/* Avoid overrun */
>  			data->block[0] = 256 - command;
>  		len = data->block[0];

Thanks,
-- 
Jean Delvare
SUSE L3 Support