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

From: Yu-cheng Yu
Date: Mon Jun 10 2019 - 16:39:43 EST

On Mon, 2019-06-10 at 12:52 -0700, Dave Hansen wrote:
> On 6/10/19 12:38 PM, Yu-cheng Yu wrote:
> > > > When an application starts, its highest stack address is determined.
> > > > It uses that as the maximum the bitmap needs to cover.
> > >
> > > Huh, I didn't think we ran code from the stack. ;)
> > >
> > > Especially given the way that we implemented the new 5-level-paging
> > > address space, I don't think that expecting code to be below the stack
> > > is a good universal expectation.
> >
> > Yes, you make a good point. However, allowing the application manage the
> > bitmap
> > is the most efficient and flexible. If the loader finds a legacy lib is
> > beyond
> > the bitmap can cover, it can deal with the problem by moving the lib to a
> > lower
> > address; or re-allocate the bitmap.
> How could the loader reallocate the bitmap and coordinate with other
> users of the bitmap?

Assuming the loader actually chooses to re-allocate, it can copy the old bitmap
over to the new before doing the switch. But, I agree, the other choice is
easier; the loader can simply put the lib at lower address. AFAIK, the loader
does not request high address in mmap().

> > If the loader cannot allocate a big bitmap to cover all 5-level
> > address space (the bitmap will be large), it can put all legacy lib's
> > at lower address. We cannot do these easily in the kernel.
> This is actually an argument to do it in the kernel. The kernel can
> always allocate the virtual space however it wants, no matter how large.
> If we hide the bitmap behind a kernel API then we can put it at high
> 5-level user addresses because we also don't have to worry about the
> high bits confusing userspace.

We actually tried this. The kernel needs to reserve the bitmap space in the
beginning for every CET-enabled app, regardless of actual needs. On each memory
request, the kernel then must consider a percentage of allocated space in its
calculation, and on systems with less memory this quickly becomes a problem.