MIME-Version: 1.0
From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 24 Oct 2014 17:22:31 -0700
Message-ID: <CALCETrUmA+5499fEqp7S-nfSBVxTEo76mA9r32zYQ41E3c+YhQ@mail.gmail.com>
Subject: vmalloced stacks on x86_64?
To: "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org

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.

 - 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.

--Andy
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