From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f200.google.com (mail-pf0-f200.google.com [209.85.192.200]) by kanga.kvack.org (Postfix) with ESMTP id 076C26B0069 for ; Thu, 23 Nov 2017 10:56:02 -0500 (EST) Received: by mail-pf0-f200.google.com with SMTP id c83so17282514pfj.11 for ; Thu, 23 Nov 2017 07:56:01 -0800 (PST) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id j25si17614689pfh.255.2017.11.23.07.56.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 23 Nov 2017 07:56:00 -0800 (PST) Received: from mail-it0-f41.google.com (mail-it0-f41.google.com [209.85.214.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 215FA219A0 for ; Thu, 23 Nov 2017 15:56:00 +0000 (UTC) Received: by mail-it0-f41.google.com with SMTP id 187so8812278iti.5 for ; Thu, 23 Nov 2017 07:56:00 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: <20171123003438.48A0EEDE@viggo.jf.intel.com> <20171123003459.C0FF167A@viggo.jf.intel.com> From: Andy Lutomirski Date: Thu, 23 Nov 2017 07:55:38 -0800 Message-ID: Subject: Re: [PATCH 11/23] x86, kaiser: map entry stack variables Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: Andy Lutomirski , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , moritz.lipp@iaik.tugraz.at, Daniel Gruss , michael.schwarz@iaik.tugraz.at, richard.fellner@student.tugraz.at, Linus Torvalds , Kees Cook , Hugh Dickins , X86 ML On Thu, Nov 23, 2017 at 7:37 AM, Dave Hansen wrote: > On 11/22/2017 07:31 PM, Andy Lutomirski wrote: >> On Wed, Nov 22, 2017 at 4:34 PM, Dave Hansen >> wrote: >>> >>> From: Dave Hansen >>> >>> There are times where the kernel is entered but there is not a >>> safe stack, like at SYSCALL entry. To obtain a safe stack, the >>> per-cpu variables 'rsp_scratch' and 'cpu_current_top_of_stack' >>> are used to save the old %rsp value and to find where the kernel >>> stack should start. >>> >>> You can not directly manipulate the CR3 register. You can only >>> 'MOV' to it from another register, which means a register must be >>> clobbered in order to do any CR3 manipulation. User-mapping >>> these variables allows us to obtain a safe stack and use it for >>> temporary storage *before* CR3 is switched. >>> >>> Signed-off-by: Dave Hansen >>> Cc: Moritz Lipp >>> Cc: Daniel Gruss >>> Cc: Michael Schwarz >>> Cc: Richard Fellner >>> Cc: Andy Lutomirski >>> Cc: Linus Torvalds >>> Cc: Kees Cook >>> Cc: Hugh Dickins >>> Cc: x86@kernel.org >>> --- >>> >>> b/arch/x86/kernel/cpu/common.c | 2 +- >>> b/arch/x86/kernel/process_64.c | 2 +- >>> 2 files changed, 2 insertions(+), 2 deletions(-) >>> >>> diff -puN arch/x86/kernel/cpu/common.c~kaiser-user-map-stack-helper-vars arch/x86/kernel/cpu/common.c >>> --- a/arch/x86/kernel/cpu/common.c~kaiser-user-map-stack-helper-vars 2017-11-22 15:45:50.128619736 -0800 >>> +++ b/arch/x86/kernel/cpu/common.c 2017-11-22 15:45:50.134619736 -0800 >>> @@ -1524,7 +1524,7 @@ EXPORT_PER_CPU_SYMBOL(__preempt_count); >>> * the top of the kernel stack. Use an extra percpu variable to track the >>> * top of the kernel stack directly. >>> */ >>> -DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) = >>> +DEFINE_PER_CPU_USER_MAPPED(unsigned long, cpu_current_top_of_stack) = >>> (unsigned long)&init_thread_union + THREAD_SIZE; >> >> This is in an x86_32-only section and should be dropped, I think. > > It's used in entry_SYSCALL_64 (see below). But I do think it's safe to > drop now. We switch before we use it. > >>> diff -puN arch/x86/kernel/process_64.c~kaiser-user-map-stack-helper-vars arch/x86/kernel/process_64.c >>> --- a/arch/x86/kernel/process_64.c~kaiser-user-map-stack-helper-vars 2017-11-22 15:45:50.130619736 -0800 >>> +++ b/arch/x86/kernel/process_64.c 2017-11-22 15:45:50.134619736 -0800 >>> @@ -59,7 +59,7 @@ >>> #include >>> #endif >>> >>> -__visible DEFINE_PER_CPU(unsigned long, rsp_scratch); >>> +__visible DEFINE_PER_CPU_USER_MAPPED(unsigned long, rsp_scratch); >>> >> This shouldn't be needed any more either. > > What about this hunk? It touches rsp_scratch before switching: > > @@ -207,9 +210,16 @@ ENTRY(entry_SYSCALL_64) > > swapgs > movq %rsp, PER_CPU_VAR(rsp_scratch) > - movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp > > - TRACE_IRQS_OFF > + /* > + * The kernel CR3 is needed to map the process stack, but we > + * need a scratch register to be able to load CR3. %rsp is > + * clobberable right now, so use it as a scratch register. > + * %rsp will be look crazy here for a couple instructions. > + */ > + SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp > + > + movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp > > I'm surprised that boots, since that hunk won't execute at all. I think you should move that code into the trampoline. (Check my latest tree -- I think it's a bit off in Ingo's tree.) I've effectively split SYSCALL64 into two separate paths: entry_SYSCALL_64 (with stack switching off) and entry_SYSCALL_64_trampoline (with stack switching on). The entire point of the trampoline was to get a way to access some data that varies per cpu without needing access to traditional %gs-based percpu data. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org