Commit 3f41b609 authored Feb 20, 2019 by Andrey Konovalov Committed by Linus Torvalds Feb 21, 2019

kasan: fix random seed generation for tag-based mode

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>

parent 1062af92

arch/arm64/kernel/setup.c

+3 −0

Original line number	Diff line number	Diff line
		@@ -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

arch/arm64/mm/kasan_init.c

+0 −2

Original line number	Diff line number	Diff line
		@@ -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");

mm/kasan/tags.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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();
		}

		/*

Admin message