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

[Chuck Ebbert]

On Wed, 1 Oct 2014 09:09:13 -0500
Chuck Ebbert <cebbert.lkml@xxxxxxxxx> wrote:

> On Tue, 30 Sep 2014 21:51:27 -0700
> 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.
> > 
> > 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@xxxxxxxxxxxxxxx
> > Reported-by: Anish Bhatt <anish@xxxxxxxxxxx>
> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > ---
> >  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.
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