arm64: Implement thread_struct whitelist for hardened usercopy

While ARM64 carries FPU state in the thread structure that is saved and
restored during signal handling, it doesn't need to declare a usercopy
whitelist, since existing accessors are all either using a bounce buffer
(for which whitelisting isn't checking the slab), are statically sized
(which will bypass the hardened usercopy check), or both.

Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Morse <[email protected]>
Cc: "Peter Zijlstra (Intel)" <[email protected]>
Cc: Dave Martin <[email protected]>
Cc: zijun_hu <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>

arch/arm64/Kconfig

@@ -90,6 +90,7 @@ config ARM64
 	select HAVE_ARCH_MMAP_RND_BITS
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
 	select HAVE_ARCH_SECCOMP_FILTER
+	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
 	select HAVE_ARCH_VMAP_STACK

arch/arm64/include/asm/processor.h

@@ -113,6 +113,16 @@ struct thread_struct {
 	struct debug_info	debug;		/* debugging */
 };
 
+/*
+ * Everything usercopied to/from thread_struct is statically-sized, so
+ * no hardened usercopy whitelist is needed.
+ */
+static inline void arch_thread_struct_whitelist(unsigned long *offset,
+						unsigned long *size)
+{
+	*offset = *size = 0;
+}
+
 #ifdef CONFIG_COMPAT
 #define task_user_tls(t)						\
 ({									\