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

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Date:	Fri, 24 Apr 2015 13:57:58 -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 1:41 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Apr 24, 2015 at 10:33 AM, Brian Gerst <brgerst@...il.com> wrote:
>>
>> To clarify, I was thinking of the CONFIG_PREEMPT case.  A nested
>> interrupt wouldn't change SS, and IST interrupts can't schedule.
>
> It has absolutely nothing to do with nested interrupts or CONFIG_PREEMPT.
>
> The problem happens simply because
>
>  - process A does a system call  SS=__KERNEL_DS
>
>  - the system call sleeps for whatever reason. SS is still __KERNEL_DS
>
>  - process B runs, returns to user space, and takes an interrupt. Now SS=0
>
>  - process B is about to return to user space (where the interrupt
> happened), but we schedule as part of that regular user-space return.
> SS=0
>
>  - process A returns to user space using sysret, the SS selector
> becomes __USER_DS, but the cached descriptor remains non-present
>
> Notice? No nested interrupts, no CONFIG_PREEMPT, nothing special at all.
>
> The reason Luto's patch fixes the problem is that now the scheduling
> from B back to A will reload SS, making it __KERNEL_DS, but more
> importantly, fixing the cached descriptor to be the usual present flag
> one, which is what the AMD sysret instruction needs.
>
> Or do I misunderstand what you are talking about?
>
>                        Linus

Your explanation is correct.  I meant that this can happen even if
CONFIG_PREEMPT is disabled.  I just took "preemption" to mean kernel
preemption, not normal scheduling.

--
Brian Gerst
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