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

[Andy Lutomirski]

On Thu, Apr 23, 2015 at 8:22 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Apr 23, 2015 at 5:34 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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.

How about, in 64-bit code: syscall; long jump to a 32-bit cs; mov with
ss override (or push or pop).

>
> 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?

See other thread.  I bet we got poisoned by a concurrently running,
preempted 16-bit or segmented process.

--Andy
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