kasan: fix random seed generation for tag-based mode

xairy · torvalds · commit 3f41b6093823 · 2019-02-21T09:01:00.000-08:00
There are two issues with assigning random percpu seeds right now: 1. We use for_each_possible_cpu() to iterate over cpus, but cpumask is not set up yet at the moment of kasan_init(), and thus we only set the seed for cpu #0. 2. A call to get_random_u32() always returns the same number and produces a message in dmesg, since the random subsystem is not yet initialized. Fix 1 by calling kasan_init_tags() after cpumask is set up. Fix 2 by using get_cycles() instead of get_random_u32(). This gives us lower quality random numbers, but it's good enough, as KASAN is meant to be used as a debugging tool and not a mitigation. Link: http://lkml.kernel.org/r/1f815cc914b61f3516ed4cc9bfd9eeca9bd5d9de.1550677973.git.andreyknvl@google.com Signed-off-by: Andrey Konovalov <andreyknvl@google.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Alexander Potapenko <glider@google.com> Cc: Dmitry Vyukov <dvyukov@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
@@ -339,6 +339,9 @@ void __init 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_tags();
+
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 	/*
 	 * Make sure init_thread_info.ttbr0 always generates translation
diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
@@ -252,8 +252,6 @@ void __init kasan_init(void)
 	memset(kasan_early_shadow_page, KASAN_SHADOW_INIT, PAGE_SIZE);
 	cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
 
-	kasan_init_tags();
-
 	/* At this point kasan is fully initialized. Enable error messages */
 	init_task.kasan_depth = 0;
 	pr_info("KernelAddressSanitizer initialized\n");
diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
@@ -46,7 +46,7 @@ void kasan_init_tags(void)
 	int cpu;
 
 	for_each_possible_cpu(cpu)
-		per_cpu(prng_state, cpu) = get_random_u32();
+		per_cpu(prng_state, cpu) = (u32)get_cycles();
 }
 
 /*

Original file line number	Diff line number	Diff line change
`@@ -46,7 +46,7 @@ void kasan_init_tags(void)`
`46`	`46`	`int cpu;`
`47`	`47`
`48`	`48`	`for_each_possible_cpu(cpu)`
`49`		`- per_cpu(prng_state, cpu) = get_random_u32();`
	`49`	`+ per_cpu(prng_state, cpu) = (u32)get_cycles();`
`50`	`50`	`}`
`51`	`51`
`52`	`52`	`/*`