linux-kernel - Re: [PATCH v2] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

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Message-ID: <11A69223-B473-4A4B-AD39-7D54381CE34D@zytor.com>
Date:	Sun, 26 Apr 2015 18:31:42 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>
CC:	the arch/x86 maintainers <x86@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	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 v2] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

In case it matters:

Acked-by: H. Peter Anvin <hpa@...or.com>

On April 26, 2015 5:51:26 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>Just a heads-up to the x86 people: I'm going to merge this directly,
>since I'm doing -rc1 momentarily, and without this patch 32-bit
>user-land on a 64-bit kernel is flaky on all AMD CPU's. Which I don't
>want for -rc1.
>
>                         Linus
>
>On Sun, Apr 26, 2015 at 4:47 PM, Andy Lutomirski <luto@...nel.org>
>wrote:
>> AMD CPUs don't reinitialize the SS descriptor on SYSRET, so SYSRET
>> with SS == 0 results in an invalid usermode state in which SS is
>> apparently equal to __USER_DS but causes #SS if used.
>>
>> Work around the issue by setting SS to __KERNEL_DS __switch_to, thus
>> ensuring that SYSRET never happens with SS set to NULL.

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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