Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

[Peter Zijlstra]

On Thu, Jun 06, 2019 at 01:09:15PM -0700, Yu-cheng Yu wrote:
> Indirect Branch Tracking (IBT) provides an optional legacy code bitmap
> that allows execution of legacy, non-IBT compatible library by an
> IBT-enabled application.  When set, each bit in the bitmap indicates
> one page of legacy code.
> 
> The bitmap is allocated and setup from the application.

> +int cet_setup_ibt_bitmap(unsigned long bitmap, unsigned long size)
> +{
> +	u64 r;
> +
> +	if (!current->thread.cet.ibt_enabled)
> +		return -EINVAL;
> +
> +	if (!PAGE_ALIGNED(bitmap) || (size > TASK_SIZE_MAX))
> +		return -EINVAL;
> +
> +	current->thread.cet.ibt_bitmap_addr = bitmap;
> +	current->thread.cet.ibt_bitmap_size = size;
> +
> +	/*
> +	 * Turn on IBT legacy bitmap.
> +	 */
> +	modify_fpu_regs_begin();
> +	rdmsrl(MSR_IA32_U_CET, r);
> +	r |= (MSR_IA32_CET_LEG_IW_EN | bitmap);
> +	wrmsrl(MSR_IA32_U_CET, r);
> +	modify_fpu_regs_end();
> +
> +	return 0;
> +}

So you just program a random user supplied address into the hardware.
What happens if there's not actually anything at that address or the
user munmap()s the data after doing this?