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

From: Andy Lutomirski <luto@kernel.org>
To: Mike Rapoport <rppt@linux.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Alexandre Chartre <alexandre.chartre@oracle.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	James Bottomley <James.Bottomley@hansenpartnership.com>,
	Jonathan Adams <jwadams@google.com>,
	Kees Cook <keescook@chromium.org>, Paul Turner <pjt@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linux-MM <linux-mm@kvack.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	X86 ML <x86@kernel.org>
Subject: Re: [RFC PATCH 5/7] x86/mm/fault: hook up SCI verification
Date: Tue, 30 Apr 2019 09:44:09 -0700	[thread overview]
Message-ID: <CALCETrWrtRo1PqdVmJQQ95J8ORy9WBkUraJCqL6JNmmAkw=H0w@mail.gmail.com> (raw)
In-Reply-To: <20190428054711.GD14896@rapoport-lnx>

On Sat, Apr 27, 2019 at 10:47 PM Mike Rapoport <rppt@linux.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@linux.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.