stackprotector: actually use get_random_canary()

The RNG always mixes in the Linux version extremely early in boot. It
also always includes a cycle counter, not only during early boot, but
each and every time it is invoked prior to being fully initialized.
Together, this means that the use of additional xors inside of the
various stackprotector.h files is superfluous and over-complicated.
Instead, we can get exactly the same thing, but better, by just calling
`get_random_canary()`.

Acked-by: Guo Ren <guoren@kernel.org> # for csky
Acked-by: Catalin Marinas <catalin.marinas@arm.com> # for arm64
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

arch/arm/include/asm/stackprotector.h

@@ -15,9 +15,6 @@
 #ifndef _ASM_STACKPROTECTOR_H
 #define _ASM_STACKPROTECTOR_H 1
 
-#include <linux/random.h>
-#include <linux/version.h>
-
 #include <asm/thread_info.h>
 
 extern unsigned long __stack_chk_guard;
@@ -30,11 +27,7 @@ extern unsigned long __stack_chk_guard;
  */
 static __always_inline void boot_init_stack_canary(void)
 {
-	unsigned long canary;
-
-	/* Try to get a semi random initial value. */
-	get_random_bytes(&canary, sizeof(canary));
-	canary ^= LINUX_VERSION_CODE;
+	unsigned long canary = get_random_canary();
 
 	current->stack_canary = canary;
 #ifndef CONFIG_STACKPROTECTOR_PER_TASK