Messages in this thread | | | Date | Thu, 23 Apr 2015 08:22:41 -0700 | Subject | Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary | From | Linus Torvalds <> |
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On Thu, Apr 23, 2015 at 5:34 AM, Denys Vlasenko <dvlasenk@redhat.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.
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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