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

[Yu-cheng Yu]

On Fri, 2019-06-07 at 09:35 -0700, Andy Lutomirski wrote:
> > On Jun 7, 2019, at 9:23 AM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> 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?
> > 
> 
> One might reasonably wonder why this state is privileged in the first place
> and, given that, why weâre allowing it to be written like this.
> 
> Arguably we should have another prctl to lock these values (until exec) as a
> gardening measure.

We can prevent the bitmap from being set more than once.  I will test it.

Yu-cheng