Re: [PATCH] Harden tcache double-free check

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 06/07/2021 22:29, Siddhesh Poyarekar via Libc-alpha wrote:

> +/* Process-wide key to try and catch a double-free in the same thread.  */
> +static uintptr_t tcache_key;
> +
> +/* The value of tcache_key does not really have to be a cryptographically
> +   secure random number.  It only needs to be arbitrary enough so that it does
> +   not collide with values present in applications, which would be quite rare,
> +   about 1 in 2^wordsize.  */
> +static void
> +tcache_key_initialize (void)
> +{
> +  if (__getrandom (&tcache_key, sizeof(tcache_key), GRND_NONBLOCK)
> +      != sizeof (tcache_key))
> +    {
> +      tcache_key = random_bits ();
> +#if __WORDSIZE == 64
> +      tcache_key = (tcache_key << 32) | random_bits ();
> +#endif

The other usage for ramdom_bits at sysdeps/posix/tempname.c already uses
a uint_fast64_t, so I think it would be better to make it random_bits()
return 64-bit instead.

Also, calling random_bits() in a sequence will result in quite low
entropy, since it uses __clock_gettime64().  Maybe you could add
XOR the pid for the higher bits:

  static inline uint64_t
  random_bits (void)
  {
    struct __timespec64 tv;
    __clock_gettime64 (CLOCK_MONOTONIC, &tv);
    /* Shuffle the lower bits to minimize the clock bias.  */
    uint32_t ret_low = tv.tv_nsec ^ tv.tv_sec;
    ret_low ^= (ret_low << 24) | (ret_low >> 8);
    /* And add a lit more entropy for higher bits.  */
    uint32_t ret_high = ret_low ^ (uint32_t) __getpid ();
    return ((uint64_t) ret_high << 32) | ret_low;
  }