linux-kernel - Re: [PATCH v2 3/3] x86/entry/64: Move #BP from IST to the IRQ stack

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Date:	Thu, 30 Jul 2015 06:19:33 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, Willy Tarreau <w@....eu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 3/3] x86/entry/64: Move #BP from IST to the IRQ stack

On Wed, Jul 29, 2015 at 10:57:26AM -0700, Andy Lutomirski wrote:
> OK if I do that as a follow-up?  It would probably want to be a
> separate patch anyway.

Of course.

> Hmm, I'm starting to like this new regime in which we never ever
> switch to user mode from anywhere other than the standard kernel
> stack.  It looks like even Xen may play along and do it cleanly soon
> :)  Maybe I'll even add an assertion somewhere to make sure we don't
> break it.  (I think this also means that the bad iret fixup can be
> simplified.)

Definitely sounds like a nice, logical thing. We sometimes switch stacks
to land on the kernel stack before returning to user mode (IST and all)
but I guess that's a clean enough thing to do. Oh, and only a couple of
insns so yeah.

> Also, with all this stuff applied (and the modify_ldt thing, once the
> Xen folks figure out what's wrong), I think we can reinstate the old
> LARL check for 16-bit segments and thus prevent naughty users from
> banging on espfix using only sigreturn.

Uuh, and then only check ZF. I guess this should cover all the legacy
cases, which is nice.

Yeah, sounds coolio. :-)

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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