NOMMU: Lock i_mmap_mutex for access to the VMA prio list

Lock i_mmap_mutex for access to the VMA prio list to prevent concurrent
access.  Currently, certain parts of the mmap handling are protected by
the region mutex, but not all.
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/nommu.c

@@ -696,9 +696,11 @@ static void add_vma_to_mm(struct mm_struct *mm, struct vm_area_struct *vma)
 	if (vma->vm_file) {
 		mapping = vma->vm_file->f_mapping;
 
+		mutex_lock(&mapping->i_mmap_mutex);
 		flush_dcache_mmap_lock(mapping);
 		vma_prio_tree_insert(vma, &mapping->i_mmap);
 		flush_dcache_mmap_unlock(mapping);
+		mutex_unlock(&mapping->i_mmap_mutex);
 	}
 
 	/* add the VMA to the tree */
@@ -760,9 +762,11 @@ static void delete_vma_from_mm(struct vm_area_struct *vma)
 	if (vma->vm_file) {
 		mapping = vma->vm_file->f_mapping;
 
+		mutex_lock(&mapping->i_mmap_mutex);
 		flush_dcache_mmap_lock(mapping);
 		vma_prio_tree_remove(vma, &mapping->i_mmap);
 		flush_dcache_mmap_unlock(mapping);
+		mutex_unlock(&mapping->i_mmap_mutex);
 	}
 
 	/* remove from the MM's tree and list */
@@ -2052,6 +2056,7 @@ int nommu_shrink_inode_mappings(struct inode *inode, size_t size,
 	high = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
 
 	down_write(&nommu_region_sem);
+	mutex_lock(&inode->i_mapping->i_mmap_mutex);
 
 	/* search for VMAs that fall within the dead zone */
 	vma_prio_tree_foreach(vma, &iter, &inode->i_mapping->i_mmap,
@@ -2059,6 +2064,7 @@ int nommu_shrink_inode_mappings(struct inode *inode, size_t size,
 		/* found one - only interested if it's shared out of the page
 		 * cache */
 		if (vma->vm_flags & VM_SHARED) {
+			mutex_unlock(&inode->i_mapping->i_mmap_mutex);
 			up_write(&nommu_region_sem);
 			return -ETXTBSY; /* not quite true, but near enough */
 		}
@@ -2086,6 +2092,7 @@ int nommu_shrink_inode_mappings(struct inode *inode, size_t size,
 		}
 	}
 
+	mutex_unlock(&inode->i_mapping->i_mmap_mutex);
 	up_write(&nommu_region_sem);
 	return 0;
 }