kasan: fix shadow_size calculation error in kasan_module_alloc

There is a special case that the size is "(N << KASAN_SHADOW_SCALE_SHIFT)
Pages plus X", the value of X is [1, KASAN_SHADOW_SCALE_SIZE-1].  The
operation "size >> KASAN_SHADOW_SCALE_SHIFT" will drop X, and the
roundup operation can not retrieve the missed one page.  For example:
size=0x28006, PAGE_SIZE=0x1000, KASAN_SHADOW_SCALE_SHIFT=3, we will get
shadow_size=0x5000, but actually we need 6 pages.

  shadow_size = round_up(size >> KASAN_SHADOW_SCALE_SHIFT, PAGE_SIZE);

This can lead to a kernel crash when kasan is enabled and the value of
mod->core_layout.size or mod->init_layout.size is like above.  Because
the shadow memory of X has not been allocated and mapped.

move_module:
  ptr = module_alloc(mod->core_layout.size);
  ...
  memset(ptr, 0, mod->core_layout.size);		//crashed

  Unable to handle kernel paging request at virtual address ffff0fffff97b000
  ......
  Call trace:
    __asan_storeN+0x174/0x1a8
    memset+0x24/0x48
    layout_and_allocate+0xcd8/0x1800
    load_module+0x190/0x23e8
    SyS_finit_module+0x148/0x180

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Zhen Lei <[email protected]>
Reviewed-by: Dmitriy Vyukov <[email protected]>
Acked-by: Andrey Ryabinin <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Hanjun Guo <[email protected]>
Cc: Libin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/kasan/kasan.c

@@ -619,12 +619,13 @@ void kasan_kfree_large(void *ptr, unsigned long ip)
 int kasan_module_alloc(void *addr, size_t size)
 {
 	void *ret;
+	size_t scaled_size;
 	size_t shadow_size;
 	unsigned long shadow_start;
 
 	shadow_start = (unsigned long)kasan_mem_to_shadow(addr);
-	shadow_size = round_up(size >> KASAN_SHADOW_SCALE_SHIFT,
-			PAGE_SIZE);
+	scaled_size = (size + KASAN_SHADOW_MASK) >> KASAN_SHADOW_SCALE_SHIFT;
+	shadow_size = round_up(scaled_size, PAGE_SIZE);
 
 	if (WARN_ON(!PAGE_ALIGNED(shadow_start)))
 		return -EINVAL;