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

From: Dave Hansen <[email protected]>

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.

---

 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
@@ -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

_