fortify: Use __builtin_dynamic_object_size() when available

Since the commits starting with c37495d ("slab: add __alloc_size
attributes for better bounds checking"), the compilers have runtime
allocation size hints available in some places. This was immediately
available to CONFIG_UBSAN_BOUNDS, but CONFIG_FORTIFY_SOURCE needed
updating to explicitly make use of the hints via the associated
__builtin_dynamic_object_size() helper. Detect and use the builtin when
it is available, increasing the accuracy of the mitigation. When runtime
sizes are not available, __builtin_dynamic_object_size() falls back to
__builtin_object_size(), leaving the existing bounds checking unchanged.

Additionally update the VMALLOC_LINEAR_OVERFLOW LKDTM test to make the
hint invisible, otherwise the architectural defense is not exercised
(the buffer overflow is detected in the memset() rather than when it
crosses the edge of the allocation).

Cc: Arnd Bergmann <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Tom Rix <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Miguel Ojeda <[email protected]> # include/linux/compiler_attributes.h
Signed-off-by: Kees Cook <[email protected]>

Author: kees

drivers/misc/lkdtm/heap.c

@@ -31,6 +31,7 @@ static void lkdtm_VMALLOC_LINEAR_OVERFLOW(void)
 	char *one, *two;
 
 	one = vzalloc(PAGE_SIZE);
+	OPTIMIZER_HIDE_VAR(one);
 	two = vzalloc(PAGE_SIZE);
 
 	pr_info("Attempting vmalloc linear overflow ...\n");

include/linux/compiler_attributes.h

@@ -297,6 +297,11 @@
  *
  * clang: https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size
  */
+#if __has_attribute(__pass_dynamic_object_size__)
+# define __pass_dynamic_object_size(type)	__attribute__((__pass_dynamic_object_size__(type)))
+#else
+# define __pass_dynamic_object_size(type)
+#endif
 #if __has_attribute(__pass_object_size__)
 # define __pass_object_size(type)	__attribute__((__pass_object_size__(type)))
 #else

include/linux/fortify-string.h

@@ -90,10 +90,17 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
  * size, rather than struct size), but there remain some stragglers using
  * type 0 that will be converted in the future.
  */
+#if __has_builtin(__builtin_dynamic_object_size)
+#define POS			__pass_dynamic_object_size(1)
+#define POS0			__pass_dynamic_object_size(0)
+#define __struct_size(p)	__builtin_dynamic_object_size(p, 0)
+#define __member_size(p)	__builtin_dynamic_object_size(p, 1)
+#else
 #define POS			__pass_object_size(1)
 #define POS0			__pass_object_size(0)
 #define __struct_size(p)	__builtin_object_size(p, 0)
 #define __member_size(p)	__builtin_object_size(p, 1)
+#endif
 
 #define __compiletime_lessthan(bounds, length)	(	\
 	__builtin_constant_p((bounds) < (length)) &&	\