linux-kernel - Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32

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Date:	Thu, 23 Apr 2015 13:05:45 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS
 to %ss

On 04/23/2015 12:44 PM, Borislav Petkov wrote:
> On Thu, Apr 23, 2015 at 12:26:43PM +0200, Denys Vlasenko wrote:
>> Yes. It loads *selector*. AMD docs say that selector is loaded as you say,
>> but *cached descriptor* of SS (which is a different entity) is not modified.
>>
>> If *cached descriptor* is invalid, in 32-bit mode stack ops
>> will fail. (In 64-bit mode, CPU doesn't do those checks).
> 
> So how can that happen with wine? Something's changing the cached
> descriptor ... ?

Yes. We know of at least one case where documentation
(both Intel and AMD) specifically states that %ss is set to NULL:
this happens on every interrupt and exception.

If interrupt/exception returns to the same task with IRET, all is well:
%ss is reloaded from iret frame (both selector and cached descriptor).

However, if interrupt results in a preemption, we end up in a different
task (say, Wine), and we can return to its userspace code
with SYSRETL. *This* type of return does not reload cached descriptor.

I don't know why it happens only with Wine. Maybe it just happens
with Wine much more easily than with other 32-bit tasks?
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