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Message-ID: <20180714080159.hqp36q7fxzb2ktlq@suse.de>
Date: Sat, 14 Jul 2018 10:01:59 +0200
From: Joerg Roedel <jroedel@...e.de>
To: Andy Lutomirski <luto@...capital.net>
Cc: Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.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>,
Josh Poimboeuf <jpoimboe@...hat.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>,
Brian Gerst <brgerst@...il.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>, Pavel Machek <pavel@....cz>,
"David H . Gutteridge" <dhgutteridge@...patico.ca>
Subject: Re: [PATCH 10/39] x86/entry/32: Handle Entry from Kernel-Mode on
Entry-Stack
On Fri, Jul 13, 2018 at 11:26:54PM -0700, Andy Lutomirski wrote:
> > So based on that, I did the above because the entry-stack is a per-cpu
> > data structure and I am not sure that we always return from the exception
> > on the same CPU where we got it. Therefore the path is called
> > PARANOID_... :)
>
> But we should just be able to IRET and end up right back on the entry
> stack where we were when we got interrupted.
Yeah, but using another CPUs entry-stack is a bad idea, no? Especially
since the owning CPU might have overwritten our content there already.
> On x86_64, we *definitely* can’t schedule in NMI, MCE, or #DB because
> we’re on a percpu stack. Are you *sure* we need this patch?
I am sure we need this patch, but not 100% sure that we really can
change CPUs in this path. We are not only talking about NMI, #MC and
#DB, but also about #GP and every other exception that can happen while
writing segments registers or on iret. With this implementation we are
on the safe side for this unlikely slow-path.
Regards,
Joerg
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