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Date: Wed, 1 Oct 2014 09:32:08 -0500 From: Chuck Ebbert <cebbert.lkml@...il.com> To: Andy Lutomirski <luto@...capital.net> Cc: Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Sebastian Lackner <sebastian@...-team.de>, Anish Bhatt <anish@...lsio.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, stable@...r.kernel.org Subject: Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace On Wed, 1 Oct 2014 09:09:13 -0500 Chuck Ebbert <cebbert.lkml@...il.com> wrote: > On Tue, 30 Sep 2014 21:51:27 -0700 > Andy Lutomirski <luto@...capital.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@...r.kernel.org > > Reported-by: Anish Bhatt <anish@...lsio.com> > > Signed-off-by: Andy Lutomirski <luto@...capital.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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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