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Date:	Sat, 25 Oct 2014 22:49:25 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Richard Weinberger <richard.weinberger@...il.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: vmalloced stacks on x86_64?

On Oct 25, 2014 9:11 PM, "Frederic Weisbecker" <fweisbec@...il.com> wrote:
>
> 2014-10-25 2:22 GMT+02:00 Andy Lutomirski <luto@...capital.net>:
> > Is there any good reason not to use vmalloc for x86_64 stacks?
> >
> > The tricky bits I've thought of are:
> >
> >  - On any context switch, we probably need to probe the new stack
> > before switching to it.  That way, if it's going to fault due to an
> > out-of-sync pgd, we still have a stack available to handle the fault.
>
> Would that prevent from any further fault on a vmalloc'ed kernel
> stack? We would need to ensure that pre-faulting, say the first byte,
> is enough to sync the whole new stack entirely otherwise we risk
> another future fault and some places really aren't safely faulted.
>

I think so.  The vmalloc faults only happen when the entire top-level
page table entry is missing, and those cover giant swaths of address
space.

I don't know whether the vmalloc code guarantees not to span a pmd
(pud? why couldn't these be called pte0, pte1, pte2, etc.?) boundary.

--Andy

> >
> >  - Any time we change cr3, we may need to check that the pgd
> > corresponding to rsp is there.  If now, we need to sync it over.
> >
> >  - For simplicity, we probably want all stack ptes to be present all
> > the time.  This is fine; vmalloc already works that way.
> >
> >  - If we overrun the stack, we double-fault.  This should be easy to
> > detect: any double-fault where rsp is less than 20 bytes from the
> > bottom of the stack is a failure to deliver a non-IST exception due to
> >  a stack overflow.  The question is: what do we do if this happens?
> > We could just panic (guaranteed to work).  We could also try to
> > recover by killing the offending task, but that might be a bit
> > challenging, since we're in IST context.  We could do something truly
> > awful: increment RSP by a few hundred bytes, point RIP at do_exit, and
> > return from the double fault.
> >
> > Thoughts?  This shouldn't be all that much code.
--
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