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. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
