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

[Linus Torvalds]

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.

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?

                          Linus
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