Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751371AbbFMVaZ (ORCPT ); Sat, 13 Jun 2015 17:30:25 -0400 Received: from mail-oi0-f41.google.com ([209.85.218.41]:32779 "EHLO mail-oi0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750725AbbFMVaS (ORCPT ); Sat, 13 Jun 2015 17:30:18 -0400 MIME-Version: 1.0 In-Reply-To: References: <1434066338-6619-1-git-send-email-srinivas.pandruvada@linux.intel.com> <20150612060747.GA25024@gmail.com> <20150612075013.GA8759@gmail.com> <20150612083625.GA22760@gmail.com> <20150613070359.GB26502@gmail.com> Date: Sat, 13 Jun 2015 17:30:17 -0400 Message-ID: Subject: Re: [PATCH] x86: General protection fault after STR (32 bit systems only) From: Brian Gerst To: Andy Lutomirski Cc: Ingo Molnar , "H. Peter Anvin" , Srinivas Pandruvada , Ingo Molnar , Thomas Gleixner , Pavel Machek , "Rafael J. Wysocki" , X86 ML , "linux-pm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Denys Vlasenko , Borislav Petkov , Linus Torvalds Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3365 Lines: 85 On Sat, Jun 13, 2015 at 2:23 PM, Andy Lutomirski wrote: > On Sat, Jun 13, 2015 at 12:03 AM, Ingo Molnar wrote: >> >> * Brian Gerst wrote: >> >>> On Fri, Jun 12, 2015 at 4:36 AM, Ingo Molnar wrote: >>> > >>> > * H. Peter Anvin wrote: >>> > >>> >> %es is used implicitly by string instructions. >>> > >>> > Ok, so we are probably better off reloading ES as well early, right >>> > when we return from the firmware, just in case something does >>> > a copy before we hit the ES restore in restore_processor_state(), >>> > which is a generic C function? >>> > >>> > Something like the patch below? >>> > >>> > I also added FS/GS/SS reloading to make it complete. If this (or a variant >>> > thereof, it's still totally untested) works then we can remove the segment >>> > save/restore layer in __save/restore_processor_state(). >>> > >>> > Thanks, >>> > >>> > Ingo >>> > >>> > ===========> >>> > arch/x86/kernel/acpi/wakeup_32.S | 13 +++++++++++++ >>> > 1 file changed, 13 insertions(+) >>> > >>> > diff --git a/arch/x86/kernel/acpi/wakeup_32.S b/arch/x86/kernel/acpi/wakeup_32.S >>> > index 665c6b7d2ea9..1376a7fc21b7 100644 >>> > --- a/arch/x86/kernel/acpi/wakeup_32.S >>> > +++ b/arch/x86/kernel/acpi/wakeup_32.S >>> > @@ -61,6 +61,19 @@ ENTRY(wakeup_pmode_return) >>> > >>> > >>> > restore_registers: >>> > + /* >>> > + * In case the BIOS corrupted our segment descriptors, >>> > + * reload them to clear out any shadow descriptor >>> > + * state: >>> > + */ >>> > + movl $__USER_DS, %eax >>> > + movl %eax, %ds >>> > + movl %eax, %es >>> > + movl %eax, %fs >>> > + movl %eax, %gs >>> > + movl $__KERNEL_DS, %eax >>> > + movl %eax, %ss >>> > + >>> > movl saved_context_ebp, %ebp >>> > movl saved_context_ebx, %ebx >>> > movl saved_context_esi, %esi >>> >>> If you follow the convoluted flow of the calls in this file, wakeup_pmode_return >>> is the first thing called from the trampoline on resume, and that loads the data >>> segments with __KERNEL_DS. [...] >> >> So if wakeup_pmode_return is really the first thing called then the whole premise >> of shadow descriptor corruption goes out the window: we reload all relevant >> segment registers. > > True, but it still leaves the fact that we're loading __KERNEL_DS > instead of __USER_DS, right? So we end up in the kernel in some > context (I have no clue what context) with __KERNEL_DS loaded. It's > very easy for us to inadvertently fix it: we could return to userspace > by any means whatsoever except SYSEXIT, or we could even return back > to some preempted kernel context. > > I still think we should replace __KERNEL_DS with __USER_DS in > wakeup_pmode_return and see if the problem goes away. I'm pretty sure that's what the problem is. If you look at the sysexit path, it never reloads ds/es. It assumes they are still __USER_DS set at sysenter. The iret path does restore all the user segments. -- Brian Gerst -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/