Re: [PATCH 1/2] kernel/fork: Add support for stack-end guard page

[Daniel Axtens <dja@axtens.net>]

Mark Rutland <mark.rutland@arm.com> writes:

> On Thu, Jul 25, 2019 at 09:53:08AM +0200, Dmitry Vyukov wrote:
>> On Wed, Jul 24, 2019 at 1:21 PM Mark Rutland <mark.rutland@arm.com> wrote:
>> >
>> > On Wed, Jul 24, 2019 at 11:11:49AM +0200, Dmitry Vyukov wrote:
>> > > On Tue, Jul 23, 2019 at 6:41 PM Mark Rutland <mark.rutland@arm.com> wrote:
>> > > >
>> > > > On Fri, Jul 19, 2019 at 03:28:17PM +0200, Marco Elver wrote:
>> > > > > Enabling STACK_GUARD_PAGE helps catching kernel stack overflows immediately
>> > > > > rather than causing difficult-to-diagnose corruption. Note that, unlike
>> > > > > virtually-mapped kernel stacks, this will effectively waste an entire page of
>> > > > > memory; however, this feature may provide extra protection in cases that cannot
>> > > > > use virtually-mapped kernel stacks, at the cost of a page.
>> > > > >
>> > > > > The motivation for this patch is that KASAN cannot use virtually-mapped kernel
>> > > > > stacks to detect stack overflows. An alternative would be implementing support
>> > > > > for vmapped stacks in KASAN, but would add significant extra complexity.
>> > > >
>> > > > Do we have an idea as to how much additional complexity?
>> > >
>> > > We would need to map/unmap shadow for vmalloc region on stack
>> > > allocation/deallocation. We may need to track shadow pages that cover
>> > > both stack and an unused memory, or 2 different stacks, which are
>> > > mapped/unmapped at different times. This may have some concurrency
>> > > concerns.  Not sure what about page tables for other CPU, I've seen
>> > > some code that updates pages tables for vmalloc region lazily on page
>> > > faults. Not sure what about TLBs. Probably also some problems that I
>> > > can't thought about now.
>> >
>> > Ok. So this looks big, we this hasn't been prototyped, so we don't have
>> > a concrete idea. I agree that concurrency is likely to be painful. :)
>
>> FTR, Daniel just mailed:
>> 
>> [PATCH 0/3] kasan: support backing vmalloc space with real shadow memory
>> https://groups.google.com/forum/#!topic/kasan-dev/YuwLGJYPB4I
>> Which presumably will supersede this.
>
> Neat!
>
> I'll try to follow that, (and thanks for the Cc there), but I'm not on
> any of the lists it went to. IMO it would be nice if subsequent versions
> would be Cc'd to LKML, if that's possible. :)

Will do - apologies for the oversight.

Regards,
Daniel

> Thanks,
> Mark.