linux-kernel - Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline stack

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Date:   Wed, 17 Jan 2018 08:45:03 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Brian Gerst <brgerst@...il.com>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline
 stack

On Wed, Jan 17, 2018 at 03:14:18PM +0100, Joerg Roedel wrote:
> On Wed, Jan 17, 2018 at 06:00:07AM -0800, Brian Gerst wrote:
> > On Wed, Jan 17, 2018 at 5:57 AM, Brian Gerst <brgerst@...il.com> wrote:
> > But then again, you could take a fault on the trampoline stack if you
> > get a bad segment.  Perhaps just pushing the new stack pointer onto
> > the process stack before user segment loads will be the right move.
> 
> User segment loads pop from the stack, so having anything on-top also
> doesn't work.
> 
> Maybe I can leave some space at the bottom of the task-stack at entry
> time and store the pointer there on exit, if that doesn't confuse the
> stack unwinder too much.

If you put it at the end of the stack page, I _think_ all you'd have to
do is just adjust TOP_OF_KERNEL_STACK_PADDING.

-- 
Josh