Re: [PATCH v2] iio: accel: bmc150: clamp the device-reported FIFO frame count

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Mon, Jun 15, 2026 at 03:43:17PM -0500, Bryam Vargas via B4 Relay wrote:

> __bmc150_accel_fifo_flush() copies the number of samples the device
> reports in its hardware FIFO into an on-stack buffer
> 
> 	u16 buffer[BMC150_ACCEL_FIFO_LENGTH * 3];
> 
> which is sized for at most BMC150_ACCEL_FIFO_LENGTH (32) samples. The
> frame count is read from the FIFO_STATUS register and only masked to its
> 7 valid bits:
> 
> 	count = val & 0x7F;
> 
> so it can be 0..127. The only other limit applied to it is the optional
> caller-supplied sample budget:
> 
> 	if (samples && count > samples)
> 		count = samples;
> 
> which does not constrain count on the flush-all path (samples == 0), and
> leaves it well above 32 whenever samples is larger. count samples are
> then transferred into buffer[]:
> 
> 	bmc150_accel_fifo_transfer(data, (u8 *)buffer, count);
> 
> bmc150_accel_fifo_transfer() reads count * 6 bytes through regmap, so a
> malfunctioning, malicious or counterfeit accelerometer (or an attacker
> tampering with the I2C/SPI bus) that reports up to 127 frames writes up
> to 762 bytes into the 192-byte buffer: a stack out-of-bounds write of up
> to 570 bytes that clobbers the stack canary, saved registers and the
> return address.
> 
> Clamp count to BMC150_ACCEL_FIFO_LENGTH, the number of samples buffer[]
> is sized for, before the transfer, mirroring the watermark clamp already
> done in bmc150_accel_set_watermark(). A well-formed flush reports at most
> BMC150_ACCEL_FIFO_LENGTH frames, so legitimate devices are unaffected.

...

>  	if (samples && count > samples)
>  		count = samples;
>  
> +	count = min(count, BMC150_ACCEL_FIFO_LENGTH);

Can actually the above be written as

	if (samples)
		count = min3(count, samples, BMC150_ACCEL_FIFO_LENGTH);
	else
		count = min(count, BMC150_ACCEL_FIFO_LENGTH);

>  	ret = bmc150_accel_fifo_transfer(data, (u8 *)buffer, count);
>  	if (ret)
>  		return ret;

-- 
With Best Regards,
Andy Shevchenko