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

arch/arm64/include/asm/stackprotector.h

@@ -13,8 +13,6 @@
 #ifndef __ASM_STACKPROTECTOR_H
 #define __ASM_STACKPROTECTOR_H
 
-#include <linux/random.h>
-#include <linux/version.h>
 #include <asm/pointer_auth.h>
 
 extern unsigned long __stack_chk_guard;
@@ -28,12 +26,7 @@ extern unsigned long __stack_chk_guard;
 static __always_inline void boot_init_stack_canary(void)
 {
 #if defined(CONFIG_STACKPROTECTOR)
-	unsigned long canary;
-
-	/* Try to get a semi random initial value. */
-	get_random_bytes(&canary, sizeof(canary));
-	canary ^= LINUX_VERSION_CODE;
-	canary &= CANARY_MASK;
+	unsigned long canary = get_random_canary();
 
 	current->stack_canary = canary;
 	if (!IS_ENABLED(CONFIG_STACKPROTECTOR_PER_TASK))

arch/csky/include/asm/stackprotector.h

@@ -2,9 +2,6 @@
 #ifndef _ASM_STACKPROTECTOR_H
 #define _ASM_STACKPROTECTOR_H 1
 
-#include <linux/random.h>
-#include <linux/version.h>
-
 extern unsigned long __stack_chk_guard;
 
 /*
@@ -15,12 +12,7 @@ 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;
-	canary &= CANARY_MASK;
+	unsigned long canary = get_random_canary();
 
 	current->stack_canary = canary;
 	__stack_chk_guard = current->stack_canary;

arch/mips/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>
-
 extern unsigned long __stack_chk_guard;
 
 /*
@@ -28,11 +25,7 @@ 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;
 	__stack_chk_guard = current->stack_canary;

arch/powerpc/include/asm/stackprotector.h

@@ -7,8 +7,6 @@
 #ifndef _ASM_STACKPROTECTOR_H
 #define _ASM_STACKPROTECTOR_H
 
-#include <linux/random.h>
-#include <linux/version.h>
 #include <asm/reg.h>
 #include <asm/current.h>
 #include <asm/paca.h>
@@ -21,13 +19,7 @@
  */
 static __always_inline void boot_init_stack_canary(void)
 {
-	unsigned long canary;
-
-	/* Try to get a semi random initial value. */
-	canary = get_random_canary();
-	canary ^= mftb();
-	canary ^= LINUX_VERSION_CODE;
-	canary &= CANARY_MASK;
+	unsigned long canary = get_random_canary();
 
 	current->stack_canary = canary;
 #ifdef CONFIG_PPC64

arch/riscv/include/asm/stackprotector.h

@@ -3,9 +3,6 @@
 #ifndef _ASM_RISCV_STACKPROTECTOR_H
 #define _ASM_RISCV_STACKPROTECTOR_H
 
-#include <linux/random.h>
-#include <linux/version.h>
-
 extern unsigned long __stack_chk_guard;
 
 /*
@@ -16,12 +13,7 @@ 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;
-	canary &= CANARY_MASK;
+	unsigned long canary = get_random_canary();
 
 	current->stack_canary = canary;
 	if (!IS_ENABLED(CONFIG_STACKPROTECTOR_PER_TASK))

arch/sh/include/asm/stackprotector.h

@@ -2,9 +2,6 @@
 #ifndef __ASM_SH_STACKPROTECTOR_H
 #define __ASM_SH_STACKPROTECTOR_H
 
-#include <linux/random.h>
-#include <linux/version.h>
-
 extern unsigned long __stack_chk_guard;
 
 /*
@@ -15,12 +12,7 @@ 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;
-	canary &= CANARY_MASK;
+	unsigned long canary = get_random_canary();
 
 	current->stack_canary = canary;
 	__stack_chk_guard = current->stack_canary;

arch/x86/include/asm/stackprotector.h

@@ -34,7 +34,6 @@
 #include <asm/percpu.h>
 #include <asm/desc.h>
 
-#include <linux/random.h>
 #include <linux/sched.h>
 
 /*
@@ -50,22 +49,11 @@
  */
 static __always_inline void boot_init_stack_canary(void)
 {
-	u64 canary;
-	u64 tsc;
+	unsigned long canary = get_random_canary();
 
 #ifdef CONFIG_X86_64
 	BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
 #endif
-	/*
-	 * We both use the random pool and the current TSC as a source
-	 * of randomness. The TSC only matters for very early init,
-	 * there it already has some randomness on most systems. Later
-	 * on during the bootup the random pool has true entropy too.
-	 */
-	get_random_bytes(&canary, sizeof(canary));
-	tsc = rdtsc();
-	canary += tsc + (tsc << 32UL);
-	canary &= CANARY_MASK;
 
 	current->stack_canary = canary;
 #ifdef CONFIG_X86_64

arch/xtensa/include/asm/stackprotector.h

@@ -14,9 +14,6 @@
 #ifndef _ASM_STACKPROTECTOR_H
 #define _ASM_STACKPROTECTOR_H 1
 
-#include <linux/random.h>
-#include <linux/version.h>
-
 extern unsigned long __stack_chk_guard;
 
 /*
@@ -27,11 +24,7 @@ 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;
 	__stack_chk_guard = current->stack_canary;