[PATCH] x86/kpti: WARN if kernel memory mapped to userspace is non-global

[Dave Hansen <dave.hansen@linux.intel.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

All memory being mapped out to userspace is shared.  That means that
it is both safe and _expected_ to have the page table Global bit set.

If a PMD is found without Global set, it either a place that could be
performing better, or something unexpected is being mapped out to
userspace.  Both of those are things for which a warning is good.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
---

 b/arch/x86/mm/kpti.c |    3 +++
 1 file changed, 3 insertions(+)

diff -puN arch/x86/mm/kpti.c~kaiser-set-global-in-kernel-for-shared arch/x86/mm/kpti.c
--- a/arch/x86/mm/kpti.c~kaiser-set-global-in-kernel-for-shared	2017-12-15 09:47:52.884717268 -0800
+++ b/arch/x86/mm/kpti.c	2017-12-15 10:00:39.134715357 -0800
@@ -157,6 +157,9 @@ kpti_clone_pmds(unsigned long start, uns
 		if (WARN_ON(!target_pmd))
 			return;
 
+		/* Only clone PMDs which we *intend* to share: */
+		WARN_ON_ONCE(!(pmd_flags(*target_pmd) & _PAGE_GLOBAL));
+
 		/*
 		 * Copy the PMD.  That is, the kernelmode and usermode
 		 * tables will share the last-level page tables of this
_