[RFC PATCH v1 40/50] arch/*/include/asm/stackprotector.h: Use get_random_canary() consistently

[George Spelvin]

... in boot_init_stack_canary().

This is the archetypical example of where the extra security of
get_random_bytes() is wasted.  The canary is only important as
long as it's stored in __stack_chk_guard.

It's also a great example of code that has been copied around
a lot and not updated.

Remove the XOR with LINUX_VERSION_CODE as it's pointless; the inclusion
of utsname() in init_std_data in the random seeding obviates it.

The XOR with the TSC on x86 and mtfb() on powerPC were left in,
as I haven't proved them redundant yet.  For those, we call
get_random_long(), xor, and mask manually.

FUNCTIONAL CHANGE: mips and xtensa were changed from 64-bit
get_random_long() to 56-bit get_random_canary() to match the
others, in accordance with the logic in CANARY_MASK.

(We could do 1 bit better and zero *one* of the two high bytes.)

Signed-off-by: George Spelvin <lkml@xxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Cc: Paul Burton <paulburton@xxxxxxxxxx>
Cc: James Hogan <jhogan@xxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxxx
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Cc: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx>
Cc: Rich Felker <dalias@xxxxxxxx>
Cc: linux-sh@xxxxxxxxxxxxxxx
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc:  "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Chris Zankel <chris@xxxxxxxxxx>
Cc: Max Filippov <jcmvbkbc@xxxxxxxxx>
Cc: linux-xtensa@xxxxxxxxxxxxxxxx
---
 arch/arm/include/asm/stackprotector.h     | 9 +++------
 arch/arm64/include/asm/stackprotector.h   | 8 ++------
 arch/mips/include/asm/stackprotector.h    | 7 ++-----
 arch/powerpc/include/asm/stackprotector.h | 6 ++----
 arch/sh/include/asm/stackprotector.h      | 8 ++------
 arch/x86/include/asm/stackprotector.h     | 4 ++--
 arch/xtensa/include/asm/stackprotector.h  | 7 ++-----
 7 files changed, 15 insertions(+), 34 deletions(-)

diff --git a/arch/arm/include/asm/stackprotector.h b/arch/arm/include/asm/stackprotector.h
index 72a20c3a0a90b..88c66fec1b5f4 100644
--- a/arch/arm/include/asm/stackprotector.h
+++ b/arch/arm/include/asm/stackprotector.h
@@ -30,17 +30,14 @@ 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
-	__stack_chk_guard = current->stack_canary;
+	__stack_chk_guard = canary;
 #else
-	current_thread_info()->stack_canary = current->stack_canary;
+	current_thread_info()->stack_canary = canary;
 #endif
 }
 
diff --git a/arch/arm64/include/asm/stackprotector.h b/arch/arm64/include/asm/stackprotector.h
index 5884a2b028277..705f60b9df85e 100644
--- a/arch/arm64/include/asm/stackprotector.h
+++ b/arch/arm64/include/asm/stackprotector.h
@@ -26,16 +26,12 @@ 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))
-		__stack_chk_guard = current->stack_canary;
+		__stack_chk_guard = canary;
 }
 
 #endif	/* _ASM_STACKPROTECTOR_H */
diff --git a/arch/mips/include/asm/stackprotector.h b/arch/mips/include/asm/stackprotector.h
index 68d4be9e12547..6d1e4652152bc 100644
--- a/arch/mips/include/asm/stackprotector.h
+++ b/arch/mips/include/asm/stackprotector.h
@@ -28,14 +28,11 @@ 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;
+	__stack_chk_guard = canary;
 }
 
 #endif	/* _ASM_STACKPROTECTOR_H */
diff --git a/arch/powerpc/include/asm/stackprotector.h b/arch/powerpc/include/asm/stackprotector.h
index 1c8460e235838..76577b72ef736 100644
--- a/arch/powerpc/include/asm/stackprotector.h
+++ b/arch/powerpc/include/asm/stackprotector.h
@@ -21,12 +21,10 @@
  */
 static __always_inline void boot_init_stack_canary(void)
 {
-	unsigned long canary;
-
 	/* Try to get a semi random initial value. */
-	canary = get_random_canary();
+	unsigned long canary = get_random_long();
+
 	canary ^= mftb();
-	canary ^= LINUX_VERSION_CODE;
 	canary &= CANARY_MASK;
 
 	current->stack_canary = canary;
diff --git a/arch/sh/include/asm/stackprotector.h b/arch/sh/include/asm/stackprotector.h
index 35616841d0a1c..a9ef619c8a0ec 100644
--- a/arch/sh/include/asm/stackprotector.h
+++ b/arch/sh/include/asm/stackprotector.h
@@ -15,15 +15,11 @@ 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;
+	__stack_chk_guard = canary;
 }
 
 #endif /* __ASM_SH_STACKPROTECTOR_H */
diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 91e29b6a86a5e..af74fd3130cf4 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -72,9 +72,9 @@ static __always_inline void boot_init_stack_canary(void)
 	 * 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));
+	canary = get_random_u64();
 	tsc = rdtsc();
-	canary += tsc + (tsc << 32UL);
+	canary += tsc + (tsc << 32);
 	canary &= CANARY_MASK;
 
 	current->stack_canary = canary;
diff --git a/arch/xtensa/include/asm/stackprotector.h b/arch/xtensa/include/asm/stackprotector.h
index e368f94fd2af3..9807fd80e5a8e 100644
--- a/arch/xtensa/include/asm/stackprotector.h
+++ b/arch/xtensa/include/asm/stackprotector.h
@@ -27,14 +27,11 @@ 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;
+	__stack_chk_guard = canary;
 }
 
 #endif	/* _ASM_STACKPROTECTOR_H */
-- 
2.26.0