nommu: fix race between ramfs truncation and shared mmap

Fix the race between the truncation of a ramfs file and an attempt to make
a shared mmap of region of that file.

The problem is that do_mmap_pgoff() calls f_op->get_unmapped_area() to
verify that the file region is made of contiguous pages and to find its
base address - but there isn't any locking to guarantee this region until
vma_prio_tree_insert() is called by add_vma_to_mm().

Note that moving the functionality into f_op->mmap() doesn't help as that
is also called before vma_prio_tree_insert().

Instead make ramfs_nommu_check_mappings() grab nommu_region_sem whilst it
does its checks. This means that this function will wait whilst mmaps
take place.

Signed-off-by: David Howells <[e-mail]>
Acked-by: Al Viro <[e-mail]>
Cc: Greg Ungerer <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/ramfs/file-nommu.c

···
 	struct vm_area_struct *vma;
 	struct prio_tree_iter iter;
 
+	down_write(&nommu_region_sem);
+
 	/* search for VMAs that fall within the dead zone */
 	vma_prio_tree_foreach(vma, &iter, &inode->i_mapping->i_mmap,
 			      newsize >> PAGE_SHIFT,
···
 			      ) {
 		/* found one - only interested if it's shared out of the page
 		 * cache */
-		if (vma->vm_flags & VM_SHARED)
+		if (vma->vm_flags & VM_SHARED) {
+			up_write(&nommu_region_sem);
 			return -ETXTBSY; /* not quite true, but near enough */
+		}
 	}
 
+	up_write(&nommu_region_sem);
 	return 0;
 }