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>

diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index d09ec76..0098493 100644 (file)
--- 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
index 4b55b15..f37a86d 100644 (file)
--- 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
index 0777649..63fca31 100644 (file)
--- 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();
 }
 
 /*