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

[Peter Zijlstra]

On Fri, Jun 07, 2019 at 09:23:43AM -0700, Yu-cheng Yu wrote:
> On Fri, 2019-06-07 at 10:08 +0200, Peter Zijlstra wrote:
> > 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?
> 
> This function checks the bitmap's alignment and size, and anything else is the
> app's responsibility.  What else do you think the kernel should check?

I've no idea what the kernel should do; since you failed to answer the
question what happens when you point this to garbage.

Does it then fault or what?