[PATCH v2 8/8] elf: Scrub and reseed the AT_RANDOM bytes after deriving the guards (BZ 34197)

[Florian Weimer]

* Adhemerval Zanella:

> +/* The stack and pointer guards have been derived from the 16 AT_RANDOM
> +   bytes pointed to by DL_RANDOM.  Scrub them first, so the guards cannot be
> +   recovered even if the refill below fails, then refill them with fresh
> +   entropy unrelated to the guards so that getauxval (AT_RANDOM) keeps
> +   returning random bytes.  */
> +static inline void __attribute__ ((always_inline))
> +_dl_reseed_random (void **dl_random)
> +{
> +  if (*dl_random == NULL)
> +    return;
> +  memset (*dl_random, '\0', 16);
> +  __asm__ __volatile__ ("" : : "r" (*dl_random) : "memory");
> +
> +  __getrandom_nocancel_nostatus_direct (*dl_random, 16, GRND_NONBLOCK);
> +  *dl_random = NULL;
> +}

I'm afraid this fails in the wrong direction: it replaces a potential
randomness leak with guaranteed predictable randomness.  (The generic
version is worse, of course).

We expose the value through getauxval (AT_RANDOM).  We shouldn't turn
that into a constant.

Thanks,
Florian