[PATCH v2] x86-64/Xen: eliminate W+X mappings

["Jan Beulich" <JBeulich@suse.com>]

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>
---
v2: Adjust comment style and indentation.
---
While I certainly dislike the added header inclusion to obtain the
prototype for get_cpu_cap(), I couldn't find a better alternative. I'm
open to suggestions.
---
 arch/x86/xen/enlighten_pv.c |    3 +++
 arch/x86/xen/mmu_pv.c       |   10 ++++++++++
 2 files changed, 13 insertions(+)

--- 4.15-rc3/arch/x86/xen/enlighten_pv.c
+++ 4.15-rc3-x86_64-Xen-avoid-W+X/arch/x86/xen/enlighten_pv.c
@@ -88,6 +88,8 @@
 #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);
@@ -1258,6 +1260,7 @@ asmlinkage __visible void __init xen_sta
 	__userpte_alloc_gfp &= ~__GFP_HIGHMEM;
 
 	/* Work out if we support NX */
+	get_cpu_cap(&boot_cpu_data);
 	x86_configure_nx();
 
 	/* Get mfn list */
--- 4.15-rc4/arch/x86/xen/mmu_pv.c
+++ 4.15-rc4-x86_64-Xen-avoid-W+X/arch/x86/xen/mmu_pv.c
@@ -1902,6 +1902,18 @@ void __init xen_setup_kernel_pagetable(p
 	/* 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))