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

Chuck Ebbert
2014-10-01T14:56:11Z

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.