On 10/01/2014 12:49 PM, Andy Lutomirski wrote:
> On Wed, Oct 1, 2014 at 11:49 AM, Andy Lutomirski <[email protected]> wrote:
>> The NT flag doesn't do anything in long mode other than causing IRET
>> to #GP. Oddly, CPL3 code can still set NT using popf.
>>
>
> [...]
>
>> +
>> + /*
>> + * Sysenter doesn't filter flags, so we need to clear NT
>> + * ourselves. To save a few cycles, we can check whether
>> + * NT was set instead of doing an unconditional popfq.
>> + */
>> + testl $X86_EFLAGS_NT,EFLAGS(%rsp) /* saved EFLAGS match cpu */
>> + jnz sysenter_fix_flags
>> +sysenter_flags_fixed:
>> +
>
> Because this thread hasn't gone on long enough:
>
> Do we need to clear IOPL here, too? With patch 2 applied, an IOPL !=
> 0 program can leak IOPL into another task. It should be cleared on
> iret, sysexit (via popf) and sysret (directly), so this shouldn't
> matter. Am I missing something?
>
> Adding IOPL to the test will add no overhead for non-iopl-using tasks,
> but it will slighly slow down 32-bit tasks that use iopl.
>
As you correctly point out, IOPL is completely irrelevant in ring 0. We
have to restore the user space flags before returning to user space, so
it shouldn't matter.
-hpa
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