linux-kernel - Re: [PATCH 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack

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Message-Id: <7AB4F269-E0E8-4290-A764-69D8605467E8@amacapital.net>
Date:   Fri, 13 Jul 2018 23:26:54 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        "David H . Gutteridge" <dhgutteridge@...patico.ca>
Subject: Re: [PATCH 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack



> On Jul 13, 2018, at 10:21 PM, Joerg Roedel <jroedel@...e.de> wrote:
> 
>> On Fri, Jul 13, 2018 at 04:31:02PM -0700, Andy Lutomirski wrote:
>> What you're really doing is keeping it available for an extra flag.
>> Please update the comment as such.  But see below.
> 
> Thanks, will do.
> 
>>> +.macro PARANOID_EXIT_TO_KERNEL_MODE
>>> +
>>> +       /*
>>> +        * Test if we entered the kernel with the entry-stack. Most
>>> +        * likely we did not, because this code only runs on the
>>> +        * return-to-kernel path.
>>> +        */
>>> +       testl   $CS_FROM_ENTRY_STACK, PT_CS(%esp)
>>> +       jz      .Lend_\@
>>> +
>>> +       /* Unlikely slow-path */
>>> +
>>> +       /* Clear marker from stack-frame */
>>> +       andl    $(~CS_FROM_ENTRY_STACK), PT_CS(%esp)
>>> +
>>> +       /* Copy the remaining task-stack contents to entry-stack */
>>> +       movl    %esp, %esi
>>> +       movl    PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %edi
>> 
>> I'm confused.  Why do we need any special handling here at all?  How
>> could we end up with the contents of the stack frame we interrupted in
>> a corrupt state?
>> 
>> I guess I don't understand why this patch is needed.
> 
> The patch is needed because we can get exceptions in kernel-mode while
> we are already on user-cr3 and entry-stack. In this case we need to
> return with user-cr3 and entry-stack to the kernel too, otherwise we
> would go to user-space with kernel-cr3.
> 
> So based on that, I did the above because the entry-stack is a per-cpu
> data structure and I am not sure that we always return from the exception
> on the same CPU where we got it. Therefore the path is called
> PARANOID_... :)

But we should just be able to IRET and end up right back on the entry stack where we were when we got interrupted.

On x86_64, we *definitely* can’t schedule in NMI, MCE, or #DB because we’re on a percpu stack. Are you *sure* we need this patch?

> 
> 
> Regards,
> 
>    Joerg
>