RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

[David Laight <David.Laight@ACULAB.COM>]

From: Reshetova, Elena
> Sent: 30 April 2019 18:51
...
> +unsigned char random_get_byte(void)
> +{
> +    struct rnd_buffer *buffer = &get_cpu_var(stack_rand_offset);
> +    unsigned char res;
> +
> +    if (buffer->byte_counter >= RANDOM_BUFFER_SIZE) {
> +        get_random_bytes(&(buffer->buffer), sizeof(buffer->buffer));
> +        buffer->byte_counter = 0;
> +    }
> +
> +    res = buffer->buffer[buffer->byte_counter];
> +    buffer->buffer[buffer->byte_counter] = 0;

If is really worth dirtying a cache line to zero data we've used?
The unused bytes following are much more interesting.

Actually if you got 'byte_counter' into a completely different
area of memory (in data that is changed more often to avoid
dirtying an extra cache line) then not zeroing the used data
would make it harder to determine which byte will be used next.

I'm also guessing that get_cpu_var() disables pre-emption?
This code could probably run 'fast and loose' and just ignore
the fact that pre-emption would have odd effects.
All it would do is perturb the randomness!

	David


> +    buffer->byte_counter ++;
> +     put_cpu_var(stack_rand_offset);
> +    return res;
> +}
> +EXPORT_SYMBOL(random_get_byte);

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