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Date:   Thu, 03 Dec 2020 10:47:51 +0100
From:   Florian Weimer <>
To:     Topi Miettinen <>
        Jann Horn <>,
        Kees Cook <>,
        Matthew Wilcox <>,
        Mike Rapoport <>,
        Linux API <>
Subject: Re: [PATCH v5] mm: Optional full ASLR for mmap(), mremap(), vdso
 and stack

* Topi Miettinen:

> +3   Additionally enable full randomization of memory mappings created
> +    with mmap(NULL, ...). With 2, the base of the VMA used for such
> +    mappings is random, but the mappings are created in predictable
> +    places within the VMA and in sequential order. With 3, new VMAs
> +    are created to fully randomize the mappings.
> +
> +    Also mremap(..., MREMAP_MAYMOVE) will move the mappings even if
> +    not necessary and the location of stack and vdso are also
> +    randomized.
> +
> +    On 32 bit systems this may cause problems due to increased VM
> +    fragmentation if the address space gets crowded.

Isn't this a bit of an understatement?  I think you'll have to restrict
this randomization to a subregion of the entire address space, otherwise
the reduction in maximum mapping size due to fragmentation will be a
problem on 64-bit architectures as well (which generally do not support
the full 64 bits for user-space addresses).

> +    On all systems, it will reduce performance and increase memory
> +    usage due to less efficient use of page tables and inability to
> +    merge adjacent VMAs with compatible attributes. In the worst case,
> +    additional page table entries of up to 4 pages are created for
> +    each mapping, so with small mappings there's considerable penalty.

The number 4 is architecture-specific, right?

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