Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 1 Oct 2014 12:49:56 -0700 | Subject | Re: [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace |
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On Wed, Oct 1, 2014 at 11:49 AM, Andy Lutomirski <luto@amacapital.net> 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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