nommu: add support for Memory Protection Units (MPU)

Some architectures (like the Blackfin arch) implement some of the
"simpler" features that one would expect out of a MMU such as memory
protection.

In our case, we actually get read/write/exec protection down to the page
boundary so processes can't stomp on each other let alone the kernel.

There is a performance decrease (which depends greatly on the workload)
however as the hardware/software interaction was not optimized at design
time.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: David Howells <dhowells@redhat.com>
Acked-by: Greg Ungerer <gerg@snapgear.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/nommu.c

@@ -33,6 +33,7 @@
 #include <asm/uaccess.h>
 #include <asm/tlb.h>
 #include <asm/tlbflush.h>
+#include <asm/mmu_context.h>
 #include "internal.h"
 
 static inline __attribute__((format(printf, 1, 2)))
@@ -622,6 +623,22 @@ static void put_nommu_region(struct vm_region *region)
 	__put_nommu_region(region);
 }
 
+/*
+ * update protection on a vma
+ */
+static void protect_vma(struct vm_area_struct *vma, unsigned long flags)
+{
+#ifdef CONFIG_MPU
+	struct mm_struct *mm = vma->vm_mm;
+	long start = vma->vm_start & PAGE_MASK;
+	while (start < vma->vm_end) {
+		protect_page(mm, start, flags);
+		start += PAGE_SIZE;
+	}
+	update_protections(mm);
+#endif
+}
+
 /*
  * add a VMA into a process's mm_struct in the appropriate place in the list
  * and tree and add to the address space's page tree also if not an anonymous
@@ -641,6 +658,8 @@ static void add_vma_to_mm(struct mm_struct *mm, struct vm_area_struct *vma)
 	mm->map_count++;
 	vma->vm_mm = mm;
 
+	protect_vma(vma, vma->vm_flags);
+
 	/* add the VMA to the mapping */
 	if (vma->vm_file) {
 		mapping = vma->vm_file->f_mapping;
@@ -703,6 +722,8 @@ static void delete_vma_from_mm(struct vm_area_struct *vma)
 
 	kenter("%p", vma);
 
+	protect_vma(vma, 0);
+
 	mm->map_count--;
 	if (mm->mmap_cache == vma)
 		mm->mmap_cache = NULL;