From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f174.google.com ([209.85.212.174]:33694 "EHLO mail-wi0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751634AbbHRPpL (ORCPT ); Tue, 18 Aug 2015 11:45:11 -0400 Received: by wijp15 with SMTP id p15so104164186wij.0 for ; Tue, 18 Aug 2015 08:45:09 -0700 (PDT) Subject: Re: [PATCH-v3.14.y 3/6] x86/nmi/64: Switch stacks on userspace NMI entry To: Thomas D , stable@vger.kernel.org References: <20150817132349.GA26797@kroah.com> <1439852125-6581-1-git-send-email-whissi@whissi.de> <1439852125-6581-4-git-send-email-whissi@whissi.de> Cc: luto@kernel.org, Linus Torvalds , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Greg Kroah-Hartman From: Jiri Slaby Message-ID: <55D35304.8050000@suse.cz> Date: Tue, 18 Aug 2015 17:45:08 +0200 MIME-Version: 1.0 In-Reply-To: <1439852125-6581-4-git-send-email-whissi@whissi.de> Content-Type: text/plain; charset=iso-8859-2 Content-Transfer-Encoding: 7bit Sender: stable-owner@vger.kernel.org List-ID: On 08/18/2015, 12:55 AM, Thomas D wrote: > From: Andy Lutomirski > > commit 9b6e6a8334d56354853f9c255d1395c2ba570e0a upstream. > > Returning to userspace is tricky: IRET can fail, and ESPFIX can > rearrange the stack prior to IRET. > > The NMI nesting fixup relies on a precise stack layout and > atomic IRET. Rather than trying to teach the NMI nesting fixup > to handle ESPFIX and failed IRET, punt: run NMIs that came from > user mode on the normal kernel stack. > > This will make some nested NMIs visible to C code, but the C > code is okay with that. > > As a side effect, this should speed up perf: it eliminates an > RDMSR when NMIs come from user mode. > > Signed-off-by: Andy Lutomirski > Reviewed-by: Steven Rostedt > Reviewed-by: Borislav Petkov > Cc: Linus Torvalds > Cc: Peter Zijlstra > Cc: Thomas Gleixner > Cc: stable@vger.kernel.org > Signed-off-by: Ingo Molnar > Signed-off-by: Greg Kroah-Hartman > --- > arch/x86/kernel/entry_64.S | 77 +++++++++++++++++++++++++++++++++++++++++++--- > 1 file changed, 73 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S > index 28b08345..bd7d8aa 100644 > --- a/arch/x86/kernel/entry_64.S > +++ b/arch/x86/kernel/entry_64.S > @@ -1715,19 +1715,88 @@ ENTRY(nmi) > * a nested NMI that updated the copy interrupt stack frame, a > * jump will be made to the repeat_nmi code that will handle the second > * NMI. > + * > + * However, espfix prevents us from directly returning to userspace > + * with a single IRET instruction. Similarly, IRET to user mode > + * can fault. We therefore handle NMIs from user space like > + * other IST entries. > */ > > /* Use %rdx as out temp variable throughout */ > pushq_cfi %rdx > CFI_REL_OFFSET rdx, 0 > > + testb $3, CS-RIP+8(%rsp) > + jz .Lnmi_from_kernel > + > + /* > + * NMI from user mode. We need to run on the thread stack, but we > + * can't go through the normal entry paths: NMIs are masked, and > + * we don't want to enable interrupts, because then we'll end > + * up in an awkward situation in which IRQs are on but NMIs > + * are off. > + */ > + > + SWAPGS > + cld > + movq %rsp, %rdx > + movq PER_CPU_VAR(kernel_stack), %rsp I think you are wasting stack space here. With kernel_stack, you should add 5*8 (KERNEL_STACK_OFFSET) to the pointer here. I.e. space for 5 registers is pre-reserved at kernel_stack already. (Or use movq instead of the 5 pushq below.) Why don't you re-use the 3.16's version anyway? > + pushq 5*8(%rdx) /* pt_regs->ss */ > + pushq 4*8(%rdx) /* pt_regs->rsp */ > + pushq 3*8(%rdx) /* pt_regs->flags */ > + pushq 2*8(%rdx) /* pt_regs->cs */ > + pushq 1*8(%rdx) /* pt_regs->rip */ > + pushq $-1 /* pt_regs->orig_ax */ > + pushq %rdi /* pt_regs->di */ > + pushq %rsi /* pt_regs->si */ > + pushq (%rdx) /* pt_regs->dx */ > + pushq %rcx /* pt_regs->cx */ > + pushq %rax /* pt_regs->ax */ > + pushq %r8 /* pt_regs->r8 */ > + pushq %r9 /* pt_regs->r9 */ > + pushq %r10 /* pt_regs->r10 */ > + pushq %r11 /* pt_regs->r11 */ > + pushq %rbx /* pt_regs->rbx */ > + pushq %rbp /* pt_regs->rbp */ > + pushq %r12 /* pt_regs->r12 */ > + pushq %r13 /* pt_regs->r13 */ > + pushq %r14 /* pt_regs->r14 */ > + pushq %r15 /* pt_regs->r15 */ regards, -- js suse labs