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SubjectRe: Avoid speculative indirect calls in kernel
On Fri, Jan 05, 2018 at 10:59:28AM +0100, Thomas Gleixner wrote:
> I've seen the insanities which were crammed into the distro kernels, which
> have sysctls and whatever, but at the same time these kernels shipped in a

Debugfs tunables only, there are no sysctl, quoting Greg:

"It's a debugfs api, it can be changed at any time, to be anything we
want, and all is fine :)"

> haste do not even boot on a specific class of machines. [..]

If you refer to the two efi_64.c and tboot.c corner case boot failures
found over the last weekend those affected upstream 4.15-rc 4.14.12
and all PTI branches in linux-tip too (perhaps less reproducible there
because of differences in old_memmap handling).

I sent you a better version of the efi_64.c fix from Jiri privately
and you still miss the tboot fix in linux-tip so you still got a boot
failure to fix there.

This is incremental with
where the "Unbreak EFI old_memmap" fix is applied.

I respinned it after doing the more correct fix in this case too (same
as the efi_64.c improvement) while leaving the attribution to the fix
to Dave as he did the hard part.

From 0c480d1eeabd56379144a4ed6b6fb24f3b84e40e Mon Sep 17 00:00:00 2001
From: Dave Hansen <>
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 <>
Cc: Ning Sun <>
Cc: Thomas Gleixner <>
Cc: Ingo Molnar <>
Cc: "H. Peter Anvin" <>
Signed-off-by: Andrea Arcangeli <>
arch/x86/kernel/tboot.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index a4eb27918ceb..75869a4b6c41 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -127,6 +127,7 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
p4d = p4d_alloc(&tboot_mm, pgd, vaddr);
if (!p4d)
return -1;
+ pgd->pgd &= ~_PAGE_NX;
pud = pud_alloc(&tboot_mm, p4d, vaddr);
if (!pud)
return -1;
If I can help and assist in any other way let me know.


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