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

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Date:	Sun, 26 Apr 2015 14:34:13 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Fri, Apr 24, 2015 at 4:18 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Apr 23, 2015 at 7:15 PM, Andy Lutomirski <luto@...nel.org> wrote:
> Even if the issue affects SYSRETQ, it could be that we don't care.  If
> the extent of the info leak is whether we context switched during a
> 64-bit syscall to a non-syscall context, then this is basically
> uninteresting.  In that case, we could either ignore the 64-bit issue
> entirely or fix it the other way: force SS to NULL on context switch
> (much faster, I presume) and fix it up before SYSRETL as Denys
> suggested.
>
> We clearly don't have a spate of crashes in programs that do SYSCALL
> from a 64-bit CS and then far jump/return to a 32-bit CS without first
> reloading SS, since this bug has been here forever.  I agree that the
> issue is ugly (if it exists in the first place), but maybe we don't
> need to fix it.

If you feel that concerned by the speed impact,
you can also disable this fix for !CONFIG_IA32_EMULATION kernels.
If kernel builder declared they don't want 32-bit userspace,
they won't do any ridiculous "I will temporarily
switch to 32-bit mode in 64-bit tasks" stuff either.
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