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

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Date:	Fri, 24 Apr 2015 10:41:07 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Brian Gerst <brgerst@...il.com>
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 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
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