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

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Message-ID: <CA+55aFwGMSfuggJ5xjXS0jdFDAOk2m5ZTRWpdcxUyXGx5Xeo_A@mail.gmail.com>
Date:	Sun, 26 Apr 2015 17:51:26 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	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

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.
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