On 10/01/2014 12:49 PM, Andy Lutomirski wrote: > 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. > 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 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html