usercopy: Make usercopy resilient against ridiculously large copies

If 'n' is so large that it's negative, we might wrap around and mistakenly
think that the copy is OK when it's not.  Such a copy would probably
crash, but just doing the arithmetic in a more simple way lets us detect
and refuse this case.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Tested-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220612213227.3881769-4-willy@infradead.org

diff --git a/mm/usercopy.c b/mm/usercopy.c
index 30a4db3cb1df40ff918616a4196b0245875cb47e..4e1da708699bbb6b9fc69496b9c380cdc6607b38 100644 (file)
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -162,27 +162,26 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
                                     bool to_user)
 {
        uintptr_t addr = (uintptr_t)ptr;
+       unsigned long offset;
        struct folio *folio;
 
        if (is_kmap_addr(ptr)) {
-               unsigned long page_end = addr | (PAGE_SIZE - 1);
-
-               if (addr + n - 1 > page_end)
-                       usercopy_abort("kmap", NULL, to_user,
-                                       offset_in_page(ptr), n);
+               offset = offset_in_page(ptr);
+               if (n > PAGE_SIZE - offset)
+                       usercopy_abort("kmap", NULL, to_user, offset, n);
                return;
        }
 
        if (is_vmalloc_addr(ptr)) {
                struct vmap_area *area = find_vmap_area(addr);
-               unsigned long offset;
 
                if (!area)
                        usercopy_abort("vmalloc", "no area", to_user, 0, n);
 
-               offset = addr - area->va_start;
-               if (addr + n > area->va_end)
+               if (n > area->va_end - addr) {
+                       offset = addr - area->va_start;
                        usercopy_abort("vmalloc", NULL, to_user, offset, n);
+               }
                return;
        }
 
@@ -195,8 +194,8 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
                /* Check slab allocator for flags and size. */
                __check_heap_object(ptr, n, folio_slab(folio), to_user);
        } else if (folio_test_large(folio)) {
-               unsigned long offset = ptr - folio_address(folio);
-               if (offset + n > folio_size(folio))
+               offset = ptr - folio_address(folio);
+               if (n > folio_size(folio) - offset)
                        usercopy_abort("page alloc", NULL, to_user, offset, n);
        }
 }