Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

Andy Lutomirski
2014-10-01T15:04:08Z

On Wed, Oct 1, 2014 at 7:56 AM, Chuck Ebbert <cebbert.lkml@gmail.com> wrote:
> On Wed, 1 Oct 2014 07:46:54 -0700
> Andy Lutomirski <luto@amacapital.net> wrote:
>
>> On Wed, Oct 1, 2014 at 7:32 AM, Chuck Ebbert <cebbert.lkml@gmail.com> wrote:
>> > On Wed, 1 Oct 2014 09:09:13 -0500
>> > Chuck Ebbert <cebbert.lkml@gmail.com> wrote:
>> >
>> >> On Tue, 30 Sep 2014 21:51:27 -0700
>> >> 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.
>> >> >
>> >> > Entry via hardware or software interrupt clears NT automatically, so
>> >> > the only relevant entries are fast syscalls.
>> >> >
>> >> > If user code causes kernel code to run with NT set, then there's at
>> >> > least some (small) chance that it could cause trouble. For example,
>> >> > user code could cause a call to EFI code with NT set, and who knows
>> >> > what would happen? Apparently some games on Wine sometimes do
>> >> > this (!), and, if an IRET return happens, they will segfault. That
>> >> > segfault cannot be handled, because signal delivery fails, too.
>> >> >
>> >> > This patch programs the CPU to clear NT on entry via SYSCALL (both
>> >> > 32-bit and 64-bit, by my reading of the AMD APM), and it clears NT
>> >> > in software on entry via SYSENTER.
>> >> >
>> >> > To save a few cycles, this borrows a trick from Jan Beulich in Xen:
>> >> > it checks whether NT is set before trying to clear it. As a result,
>> >> > it seems to have very little effect on SYSENTER performance on my
>> >> > machine.
>> >> >
>> >> > Testers beware: on Xen, SYSENTER with NT set turns into a GPF.
>> >> >
>> >> > I haven't touched anything on 32-bit kernels.
>> >> >
>> >> > The syscall mask change comes from a variant of this patch by Anish
>> >> > Bhatt.
>> >> >
>> >> > Cc: stable@vger.kernel.org
>> >> > Reported-by: Anish Bhatt <anish@chelsio.com>
>> >> > Signed-off-by: Andy Lutomirski <luto@amacapital.net>
>> >> > ---
>> >> > arch/x86/ia32/ia32entry.S | 12 ++++++++++++
>> >> > arch/x86/kernel/cpu/common.c | 2 +-
>> >> > 2 files changed, 13 insertions(+), 1 deletion(-)
>> >> >
>> >> > diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
>> >> > index 4299eb05023c..44d1dd371454 100644
>> >> > --- a/arch/x86/ia32/ia32entry.S
>> >> > +++ b/arch/x86/ia32/ia32entry.S
>> >> > @@ -151,6 +151,18 @@ ENTRY(ia32_sysenter_target)
>> >> > 1: movl (%rbp),%ebp
>> >> > _ASM_EXTABLE(1b,ia32_badarg)
>> >> > ASM_CLAC
>> >> > +
>> >> > + /*
>> >> > + * 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 */
>> >> > + jz 1f
>> >> > + pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_FIXED)
>> >> > + popfq_cfi
>> >> > +1:
>> >> > +
>> >>
>> >> I think you've gone backwards with this version. The earlier one got
>> >> some of the performance loss back by not needing to do the "cld" insn.
>> >>
>> >> You should just replace that "cld" (line 146) with
>> >>
>> >> pushfq_cfi $2
>> >> popfq_cfi
>> >>
>> >> Unfortunately I'm not set up to test that yet. But I did look at
>> >> the SDM and can't see a need to preserve any of the flags.
>> >>
>> >
>> >
>> > <sigh> that's:
>> >
>> > pushfw_cfi $0x202
>> >
>> > IF needs to stay on because we've already enabled interrupts after
>> > sysenter.
>>
>> I tried exactly this. It was much slower than the version I sent.
>>
>
> Yeah, it looks like a new paravirt op that enables interrupts and
> clears all the other flags would be the only way to do this without at
> least some impact on performance.

We have that -- it's called something like setfl.

But it still wouldn't help. It seems that cld, test, jnz is simply
much faster than popfq.

If we could fold it with the sti earlier, *maybe* that would be a win,
but then we'd also have to patch the saved flags to avoid returning to
userspace with interrupts off. (And I tried that. It still didn't
seem to be fast enough.)

--Andy

--
Andy Lutomirski
AMA Capital Management, LLC