Re: Avoid speculative indirect calls in kernel

[Andrea Arcangeli <aarcange@redhat.com>]

On Mon, Jan 08, 2018 at 09:53:02PM +0100, Thomas Gleixner wrote:
> Thanks for resending it.

Thanks to you for the PTI improvements!

Did my best to do the cleanest patch for tip, but I now figured Dave's
original comment was spot on: a _PAGE_NX clear then becomes necessary
also after pud_alloc not only after p4d_alloc.

pmd_alloc would run into the same with x86 32bit non-PAE too.

So there are two choices, either going back to one single _PAGE_NX
clear from the original Dave's original patch as below, or to add
multiple clear after each level which was my objective and is more
robust, but it may be overkill in this case. As long as it was one
line it looked a clear improvement.

Considering the caller in both cases is going to abort I guess we can
use the one liner approach as Dave and Jiri did originally.

It's up to you, doing it at each level would be more resilent in case
the caller is changed.

For the efi_64 same issue, the current tip patch will work better, but
it can still be cleaned up with pgd_efi instead of pgd_offset_k().

I got partly fooled because it worked great with 4levels, but it
wasn't ok anyway for 32bit non-PAE. Sometime it's the simpler stuff
that gets more subtle.

Andrea

>From 391517951e904cdd231dda9943c36a25a7bf01b9 Mon Sep 17 00:00:00 2001
From: Dave Hansen <dave.hansen@linux.intel.com>
Date: Sat, 6 Jan 2018 18:41:14 +0100
Subject: [PATCH 1/1] x86/kaiser/efi: unbreak tboot

This is another case similar to what EFI does: create a new set of
page tables, map some code at a low address, and jump to it.  PTI
mistakes this low address for userspace and mistakenly marks it
non-executable in an effort to make it unusable for userspace.  Undo
the poison to allow execution.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ning Sun <ning.sun@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: tboot-devel@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 arch/x86/kernel/tboot.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index a4eb27918ceb..a2486f444073 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -138,6 +138,17 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
 		return -1;
 	set_pte_at(&tboot_mm, vaddr, pte, pfn_pte(pfn, prot));
 	pte_unmap(pte);
+
+	/*
+	 * PTI poisons low addresses in the kernel page tables in the
+	 * name of making them unusable for userspace.  To execute
+	 * code at such a low address, the poison must be cleared.
+	 *
+	 * Note: 'pgd' actually gets set in p4d_alloc() _or_
+	 * pud_alloc() depending on 4/5-level paging.
+	 */
+	pgd->pgd &= ~_PAGE_NX;
+
 	return 0;
 }