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Date: Wed, 17 Jan 2018 08:45:03 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline
 stack
Message-ID: <20180117144503.62e47m6e5yyyze3d@treble>
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To: Joerg Roedel <joro@8bytes.org>
Cc: Brian Gerst <brgerst@gmail.com>, Andy Lutomirski <luto@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, Linus Torvalds <torvalds@linux-foundation.org>, Dave Hansen <dave.hansen@intel.com>, Juergen Gross <jgross@suse.com>, Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, David Laight <David.Laight@aculab.com>, Denys Vlasenko <dvlasenk@redhat.com>, Eduardo Valentin <eduval@amazon.com>, Greg KH <gregkh@linuxfoundation.org>, Will Deacon <will.deacon@arm.com>, "Liguori, Anthony" <aliguori@amazon.com>, Daniel Gruss <daniel.gruss@iaik.tugraz.at>, Hugh Dickins <hughd@google.com>, Kees Cook <keescook@google.com>, Andrea Arcangeli <aarcange@redhat.com>, Waiman Long <llong@redhat.com>, Joerg Roedel <jroedel@suse.de>

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@gmail.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

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