linux-kernel - Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary

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Date:	Thu, 23 Apr 2015 12:06:43 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	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>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary

On Thu, Apr 23, 2015 at 11:22 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Apr 23, 2015 at 5:34 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>>
>> It was observed to cause Wine crashes. Conjectured sequence of events
>> causing it is as follows:
>>
>> 1. Wine process enters kernel via syscall insn.
>> 2. Context switch to any other task.
>> 3. Interrupt or exception happens, CPU loads %ss with 0.
>>    (This happens according to both Intel and AMD docs.)
>>    %ss cached descriptor is set to "invalid" state.
>> 4. Context switch back to Wine.
>> 5. sysret to 32-bit userspace. %ss selector has correct value but its
>>    cached descriptor is still invalid.
>
> I really don't like the patch, as it just feels very hacky to me.
>
> It is a bit scary to me that apparently we leak %ss values between
> processes, so that while we run in the kernel we can randomly have the
> ss descriptor either be 0 or __KERNEL_DS.  That sounds like an
> information leak to me, even in 64-bit mode. The value of %ss may not
> *matter* in 64-bit mode, but leaking that difference between processes
> sounds nasty. I can't offhand thing of any way to actually read the
> present bit in the cached descriptor (I was thinking something like
> the "LSL" instruction, but that takes a new segment selector, not the
> segment itself), but it just smells odd to me.

So you are saying we should save and conditionally restore the
kernel's %ss during context switch?  That shouldn't be too bad.  Half
of the time you would be loading the null selector which is fast (no
GDT access, no validation).

> Also, why does this only happen with Wine? In regular 32-bit mode the
> segment valid bit in the cached descriptor should also matter. So how
> come this doesn't trigger for any 32-bit user land on a 64-bit kernel?

Probably just lack of exposure so far.  It only affects AMD cpus, and
it was just merged.  Wine is probably the most common 32-bit app
people will run on a 64-bit kernel.  I'll test something other than
Wine that is 32-bit when I get home tonight.

--
Brian Gerst
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