arm64: add prctl control for resetting ptrauth keys

Add an arm64-specific prctl to allow a thread to reinitialize its
pointer authentication keys to random values. This can be useful when
exec() is not used for starting new processes, to ensure that different
processes still have different keys.

Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/pointer_auth.h

@@ -63,6 +63,8 @@ static inline void ptrauth_keys_switch(struct ptrauth_keys *keys)
 		__ptrauth_key_install(APGA, keys->apga);
 }
 
+extern int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg);
+
 /*
  * The EL0 pointer bits used by a pointer authentication code.
  * This is dependent on TBI0 being enabled, or bits 63:56 would also apply.
@@ -86,6 +88,7 @@ do {									\
 	ptrauth_keys_switch(&(tsk)->thread_info.keys_user)
 
 #else /* CONFIG_ARM64_PTR_AUTH */
+#define ptrauth_prctl_reset_keys(tsk, arg)	(-EINVAL)
 #define ptrauth_strip_insn_pac(lr)	(lr)
 #define ptrauth_thread_init_user(tsk)
 #define ptrauth_thread_switch(tsk)