linux-kernel - Re: [RFC PATCH 5/7] x86/mm/fault: hook up SCI verification

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Date:   Tue, 30 Apr 2019 09:44:09 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Jonathan Adams <jwadams@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Paul Turner <pjt@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux-MM <linux-mm@...ck.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: [RFC PATCH 5/7] x86/mm/fault: hook up SCI verification

On Sat, Apr 27, 2019 at 10:47 PM Mike Rapoport <rppt@...ux.ibm.com> wrote:
>
> On Fri, Apr 26, 2019 at 09:42:23AM +0200, Peter Zijlstra wrote:
> > On Fri, Apr 26, 2019 at 12:45:52AM +0300, Mike Rapoport wrote:
> > > If a system call runs in isolated context, it's accesses to kernel code and
> > > data will be verified by SCI susbsytem.
> > >
> > > Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> > > ---
> > >  arch/x86/mm/fault.c | 28 ++++++++++++++++++++++++++++
> > >  1 file changed, 28 insertions(+)
> >
> > There's a distinct lack of touching do_double_fault(). It appears to me
> > that you'll instantly trigger #DF when you #PF, because the #PF handler
> > itself will not be able to run.
>
> The #PF handler is able to run. On interrupt/error entry the cr3 is
> switched to the full kernel page tables, pretty much like PTI does for
> user <-> kernel transitions. It's in the patch 3.
>
>

PeterZ meant page_fault, not do_page_fault.  In your patch, page_fault
and some of error_entry run before that magic switchover happens.  If
they're not in the page tables, you double-fault.

And don't even try to do SCI magic in the double-fault handler.  As I
understand it, the SDM and APM aren't kidding when they say that #DF
is an abort, not a fault.  There is a single case in the kernel where
we recover from #DF, and it was vetted by microcode people.