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

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Message-ID: <CAMzpN2gDdENpa0vXrCmgvkznFiNzm=z7oPybtCZt76O6j6ZqVg@mail.gmail.com>
Date:	Fri, 24 Apr 2015 13:33:01 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	"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>,
	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 12:25 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Apr 24, 2015 at 5:00 AM, Brian Gerst <brgerst@...il.com> wrote:
>>
>> So actually this isn't a preemption issue, as the NULL SS is coming
>> from an interrupt from userspace (timer tick, etc.).
>
> It *is* a preemption issue, in the sense that the interrupt that
> clears SS also then returns to user space using an "iret" that will
> properly restore it.
>
> So the only case we need to worry about is the preemption case, where
> the interrupt has caused a task switch (typically because it woke
> something up or it was the timer interrupt and the timeslice of the
> previous task is up), and we switch to another context that returns to
> user space using "sysret" instead.

To clarify, I was thinking of the CONFIG_PREEMPT case.  A nested
interrupt wouldn't change SS, and IST interrupts can't schedule.

--
Brian Gerst
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