linux-kernel - Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

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Date:	Mon, 27 Apr 2015 09:00:08 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue

On Mon, Apr 27, 2015 at 8:46 AM, Borislav Petkov <bp@...en8.de> wrote:
>
> Right, what about the false positives:

Anybody who tries to return to kernel addresses with sysret is
suspect. It's more likely to be an attack vector than anything else
(ie somebody who is trying to take advantage of a CPU bug).

I don't think there are any false positives. The only valid sysret
targets are in normal user space.

There's the "vsyscall" area, I guess, but we are actively discouraging
people from using it (it's emulated by default) and using iret to
return from it is fine if somebody ends up using it natively. It was a
mistake to have fixed addresses with known code in it, so I don't
think we should care.

We've had the inexact version for a long time, and the exact canonical
address check hasn't even hit my tree yet. I wouldn't worry about it.

And since we haven't even merged the "better check for canonical
addresses" it cannot even be a regression if we never really use it.

                       Linus
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