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

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Date:   Sat, 14 Jul 2018 07:21:10 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 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_... :)


Regards,

	Joerg