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Date:	Sun, 26 Oct 2014 05:11:51 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Richard Weinberger <richard.weinberger@...il.com>
Subject: Re: vmalloced stacks on x86_64?

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.

>
>  - 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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