ARM: make vectors page inaccessible from userspace

commit a5463cd3 upstream.

If kuser helpers are not provided by the kernel, disable user access to
the vectors page.  With the kuser helpers gone, there is no reason for
this page to be visible to userspace.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm/include/asm/page.h

@@ -142,7 +142,9 @@ extern void __cpu_copy_user_highpage(struct page *to, struct page *from,
 #define clear_page(page)	memset((void *)(page), 0, PAGE_SIZE)
 extern void copy_page(void *to, const void *from);
 
+#ifdef CONFIG_KUSER_HELPERS
 #define __HAVE_ARCH_GATE_AREA 1
+#endif
 
 #ifdef CONFIG_ARM_LPAE
 #include <asm/pgtable-3level-types.h>

arch/arm/kernel/process.c

@@ -433,6 +433,7 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
 }
 
 #ifdef CONFIG_MMU
+#ifdef CONFIG_KUSER_HELPERS
 /*
  * The vectors page is always readable from user space for the
  * atomic helpers. Insert it into the gate_vma so that it is visible
@@ -465,10 +466,14 @@ int in_gate_area_no_mm(unsigned long addr)
 {
 	return in_gate_area(NULL, addr);
 }
+#define is_gate_vma(vma)	((vma) = &gate_vma)
+#else
+#define is_gate_vma(vma)	0
+#endif
 
 const char *arch_vma_name(struct vm_area_struct *vma)
 {
-	return (vma == &gate_vma) ? "[vectors]" :
+	return is_gate_vma(vma) ? "[vectors]" :
 		(vma->vm_mm && vma->vm_start == vma->vm_mm->context.sigpage) ?
 		 "[sigpage]" : NULL;
 }

arch/arm/mm/mmu.c

@@ -1220,7 +1220,11 @@ static void __init devicemaps_init(struct machine_desc *mdesc)
 	map.pfn = __phys_to_pfn(virt_to_phys(vectors));
 	map.virtual = 0xffff0000;
 	map.length = PAGE_SIZE;
+#ifdef CONFIG_KUSER_HELPERS
 	map.type = MT_HIGH_VECTORS;
+#else
+	map.type = MT_LOW_VECTORS;
+#endif
 	create_mapping(&map);
 
 	if (!vectors_high()) {