linux-kernel - Re: [PATCH v4 15/29] x86/mm/64: Enable vmapped stacks

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Date:	Mon, 27 Jun 2016 10:28:12 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Brian Gerst <brgerst@...il.com>, Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v4 15/29] x86/mm/64: Enable vmapped stacks

On Mon, Jun 27, 2016 at 8:54 AM, Andy Lutomirski <luto@...capital.net> wrote:
>
> How about:
>
>     tsk_stack = (unsigned long)task_stack_page(tsk);
>     if (regs->rsp <= tsk_stack + 7*8 && regs->rsp > tsk_stack - PAGE_SIZE) {

I'm not at all convinced that regs->rsp will be all that reliable
under a double-fault scenario either. I'd be more inclined to trusr
cr2 than the register state.

It's true that double faults can happen for *other* reasons entirely,
and as such it's not clear that %cr2 is reliable either, but since
this is all just about a printout, I'd rather go that way anyway.

                 Linus