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

Chuck Ebbert
2014-10-01T14:32:14Z

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.