Re: [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

[Andy Lutomirski]

On Wed, Oct 1, 2014 at 11:49 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.

--Andy
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