Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

From: Andy Lutomirski
Date: Mon Mar 09 2015 - 11:10:24 EST

On Mon, Mar 9, 2015 at 8:00 AM, Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> wrote:
> On Mon, Mar 9, 2015 at 3:18 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> Do we actually need espfix on all returns to vm86 mode?
> No, the current code (and my new version) does *not* do
> espfix for vm86. It's not needed (apparently).
>>> + btl $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp)
>>> + jc restore_nocheck # VM set, not it
>> This seems useless. In vm86 mode, espfix should work fine (even if
>> pointlessly), CS won't have the two low bits set, and SS won't
>> reference the LDT because it's not a selector at all.
> You seem to suggest we can drop VM flag test.
> If we do that, the tests for CS and SS will work on bogus data.
> I.e. they will semi-randomly rouse execution through espfix.

Mmm, right. My bad, that test is needed.

> Which will probably work correctly, but IIRC espfix does crazy stuff
> which is likely to be slow.
> What we definitely should do here is at least frame this check with
> "#ifdef CONFIG_VM86".
>> That being said, what ends up in the high bits of esp when we iret to
>> vm86 mode?
> I don't know. I guess it's time to write an actual vm86 testcase :)

Ick. I can try...

Anyway, you've convinced me that your patch is good. I queued it up.

Andy Lutomirski
AMA Capital Management, LLC
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