arm64: Fix KASAN random tag seed initialization

Currently, kasan_init_sw_tags() is called before setup_per_cpu_areas(),
so per_cpu(prng_state, cpu) accesses the same address regardless of the
value of "cpu", and the same seed value gets copied to the percpu area
for every CPU. Fix this by moving the call to smp_prepare_boot_cpu(),
which is the first architecture hook after setup_per_cpu_areas().

Fixes: 3c9e3aa1 ("kasan: add tag related helper functions")
Fixes: 3f41b609 ("kasan: fix random seed generation for tag-based mode")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://lore.kernel.org/r/20240814091005.969756-1-samuel.holland@sifive.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/setup.c

@@ -355,9 +355,6 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
 	smp_init_cpus();
 	smp_build_mpidr_hash();
 
-	/* Init percpu seeds for random tags after cpus are set up. */
-	kasan_init_sw_tags();
-
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 	/*
 	 * Make sure init_thread_info.ttbr0 always generates translation

arch/arm64/kernel/smp.c

@@ -467,6 +467,8 @@ void __init smp_prepare_boot_cpu(void)
 		init_gic_priority_masking();
 
 	kasan_init_hw_tags();
+	/* Init percpu seeds for random tags after cpus are set up. */
+	kasan_init_sw_tags();
 }
 
 /*