Re: [RFC PATCH 07/14] khwasan: add tag related helper functions

[Andrey Konovalov <andreyknvl@google.com>]

On Mon, Mar 5, 2018 at 3:32 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> On Fri, Mar 02, 2018 at 08:44:26PM +0100, Andrey Konovalov wrote:
>> +static DEFINE_PER_CPU(u32, prng_state);
>> +
>> +void khwasan_init(void)
>> +{
>> +     int cpu;
>> +
>> +     for_each_possible_cpu(cpu) {
>> +             per_cpu(prng_state, cpu) = get_random_u32();
>> +     }
>> +     WRITE_ONCE(khwasan_enabled, 1);
>> +}
>> +
>> +static inline u8 khwasan_random_tag(void)
>> +{
>> +     u32 state = this_cpu_read(prng_state);
>> +
>> +     state = 1664525 * state + 1013904223;
>> +     this_cpu_write(prng_state, state);
>> +
>> +     return (u8)state;
>> +}
>
> Have you considered preemption here? Is the assumption that it happens
> sufficiently rarely that cross-contaminating the prng state isn't a
> problem?

Hi Mark!

Yes, I have. If a preemption happens between this_cpu_read and
this_cpu_write, the only side effect is that we'll give a few
allocated in different contexts objects the same tag. Sine KHWASAN is
meant to be used a probabilistic bug-detection debug feature, this
doesn't seem to have serious negative impact.

I'll add a comment about this though.

Thanks!

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