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

[Adhemerval Zanella Netto]

On 16/06/26 15:41, Florian Weimer wrote:
> * 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.
I did not fully grasp the issue here, what do you mean by 'guaranteed
predictable randomness' here?