From: Jan Beulich <JBeulich@suse.com>
Date:   Mon Dec 18 09:37:45 2017 -0700
Subject: x86-64/Xen: eliminate W+X mappings
Patch-mainline: v4.15-rc5
Git-commit: 2cc42bac1c795f75fcc062b95c6ca7ac1b84d5d8
References: bnc#1065600

A few thousand such pages are usually left around due to the re-use of
L1 tables having been provided by the hypervisor (Dom0) or tool stack
(DomU). Set NX in the direct map variant, which needs to be done in L2
due to the dual use of the re-used L1s.

For x86_configure_nx() to actually do what it is supposed to do, call
get_cpu_cap() first. This was broken by commit 4763ed4d45 ("x86, mm:
Clean up and simplify NX enablement") when switching away from the
direct EFER read.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 69b9deff7e5c..86f26ea99324 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -87,6 +87,8 @@
 #include "multicalls.h"
 #include "pmu.h"
 
+#include "../kernel/cpu/cpu.h" /* get_cpu_cap() */
+
 void *xen_initial_gdt;
 
 static int xen_cpu_up_prepare_pv(unsigned int cpu);
@@ -1249,6 +1251,7 @@ asmlinkage __visible void __init xen_start_kernel(void)
 	__userpte_alloc_gfp &= ~__GFP_HIGHMEM;
 
 	/* Work out if we support NX */
+	get_cpu_cap(&boot_cpu_data);
 	x86_configure_nx();
 
 	/* Get mfn list */
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 9d9cc3870722..7118f776cd49 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -1916,6 +1916,18 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
 	/* Graft it onto L4[511][510] */
 	copy_page(level2_kernel_pgt, l2);
 
+	/*
+	 * Zap execute permission from the ident map. Due to the sharing of
+	 * L1 entries we need to do this in the L2.
+	 */
+	if (__supported_pte_mask & _PAGE_NX) {
+		for (i = 0; i < PTRS_PER_PMD; ++i) {
+			if (pmd_none(level2_ident_pgt[i]))
+				continue;
+			level2_ident_pgt[i] = pmd_set_flags(level2_ident_pgt[i], _PAGE_NX);
+		}
+	}
+
 	/* Copy the initial P->M table mappings if necessary. */
 	i = pgd_index(xen_start_info->mfn_list);
 	if (i && i < pgd_index(__START_KERNEL_map))