arm64: sync kernel APIAKey when installing

A direct write to a APxxKey_EL1 register requires a context
synchronization event to ensure that indirect reads made by subsequent
instructions (e.g. AUTIASP, PACIASP) observe the new value.

When we initialize the boot task's APIAKey in boot_init_stack_canary()
via ptrauth_keys_switch_kernel() we miss the necessary ISB, and so there
is a window where instructions are not guaranteed to use the new APIAKey
value. This has been observed to result in boot-time crashes where
PACIASP and AUTIASP within a function used a mixture of the old and new
key values.

Fix this by having ptrauth_keys_switch_kernel() synchronize the new key
value with an ISB. At the same time, __ptrauth_key_install() is renamed
to __ptrauth_key_install_nosync() so that it is obvious that this
performs no synchronization itself.

Fixes: 28321582 ("arm64: initialize ptrauth keys for kernel booting task")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Will Deacon <will@kernel.org>
Cc: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/pointer_auth.h

@@ -47,7 +47,7 @@ static inline void ptrauth_keys_init_user(struct ptrauth_keys_user *keys)
 		get_random_bytes(&keys->apga, sizeof(keys->apga));
 }
 
-#define __ptrauth_key_install(k, v)				\
+#define __ptrauth_key_install_nosync(k, v)			\
 do {								\
 	struct ptrauth_key __pki_v = (v);			\
 	write_sysreg_s(__pki_v.lo, SYS_ ## k ## KEYLO_EL1);	\
@@ -62,8 +62,11 @@ static __always_inline void ptrauth_keys_init_kernel(struct ptrauth_keys_kernel
 
 static __always_inline void ptrauth_keys_switch_kernel(struct ptrauth_keys_kernel *keys)
 {
-	if (system_supports_address_auth())
-		__ptrauth_key_install(APIA, keys->apia);
+	if (!system_supports_address_auth())
+		return;
+
+	__ptrauth_key_install_nosync(APIA, keys->apia);
+	isb();
 }
 
 extern int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg);