Re: [PATCH v8 1/5] kasan: support backing vmalloc space with real shadow memory

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Oct 15, 2019 at 12:57:44AM +1100, Daniel Axtens wrote:
> Hi Andrey,
> 
> 
> >> +	/*
> >> +	 * Ensure poisoning is visible before the shadow is made visible
> >> +	 * to other CPUs.
> >> +	 */
> >> +	smp_wmb();
> >
> > I'm not quite understand what this barrier do and why it needed.
> > And if it's really needed there should be a pairing barrier
> > on the other side which I don't see.
> 
> Mark might be better able to answer this, but my understanding is that
> we want to make sure that we never have a situation where the writes are
> reordered so that PTE is installed before all the poisioning is written
> out. I think it follows the logic in __pte_alloc() in mm/memory.c:
> 
> 	/*
> 	 * Ensure all pte setup (eg. pte page lock and page clearing) are
> 	 * visible before the pte is made visible to other CPUs by being
> 	 * put into page tables.

Yup. We need to ensure that if a thread sees a populated shadow PTE, the
corresponding shadow memory has been zeroed. Thus, we need to ensure
that the zeroing is observed by other CPUs before we update the PTE.

We're relying on the absence of a TLB entry preventing another CPU from
loading the corresponding shadow shadow memory until its PTE has been
populated (after the zeroing is visible). Consequently there is no
barrier on the other side, and just a control-dependency (which would be
insufficient on its own).

There is a potential problem here, as Will Deacon wrote up at:

  https://lore.kernel.org/linux-arm-kernel/20190827131818.14724-1-will@kernel.org/

... in the section starting:

| *** Other architecture maintainers -- start here! ***

... whereby the CPU can spuriously fault on an access after observing a
valid PTE.

For arm64 we handle the spurious fault, and it looks like x86 would need
something like its vmalloc_fault() applying to the shadow region to
cater for this.

Thanks,
Mark.