Thread stack and heap caches - CVE-2019-1010024

[Florian Weimer]

* Vinay Kumar:

> Regarding bug related to Thread stack and heap caches (CVE-2019-1010024).
> https://sourceware.org/bugzilla/show_bug.cgi?id=22852
>
>>> One way to harden is to use a tunable for a thread stack cache, and set that to zero.
> Below change in glibc allocatestack.c file gives the expected output
> with test case. Verified on x86_64 target.
> =======================================================
> --- a/nptl/allocatestack.c
> +++ b/nptl/allocatestack.c
> @@ -186,6 +186,7 @@ get_cached_stack (size_t *sizep, void **memp)
>        struct pthread *curr;
>
>        curr = list_entry (entry, struct pthread, list);
> +      curr->stackblock_size = 0;
>        if (FREE_P (curr) && curr->stackblock_size >= size)
>         {
>           if (curr->stackblock_size == size)
> =======================================================

Does this really change the randomization?  Won't the kernel map the new
stack at a predictable address, too?

Thanks,
Florian