Re: [PATCH] MIPS: Increase default mmap randomization bits for 64-bit

[Huacai Chen]

Hi, Bingwu,

On Sun, Mar 29, 2026 at 6:57 AM Bingwu Zhang <xtex@xxxxxxxx> wrote:
>
> From: Bingwu Zhang <xtex@xxxxxxxxxxxxx>
>
> Increase mmap randomization bits on 64-bit from [12,18] to [18,20] for
> better strength.
>
> The original default, 12, means that ASLR offset has only (1 << 12) =
> 4096 possibilities. On average, it can be brute-forced in 2048 attempts.
> If a service is configured to restart automatically or can be started
> easily (e.g. execve a suid program), then trying for 4k times can be
> done in one day even when each attempt takes 20s.
> Increasing it to 18 makes brute-force much more difficult and leaves
> more time for operators to find out attacks.
I suggest to keep ARCH_MMAP_RND_BITS_MIN and only increase
ARCH_MMAP_RND_BITS_MAX. Because performance is important by default
and you can change mmap_rnd_bits at runtime for security.


Huacai

>
> On 64-bit platforms, virtual address space is cheap, so the
> randomization bits can be increased safely without disturbing userland.
>
> In 1091458d09e1 ("MIPS: Randomize mmap if randomize_va_space is set")
> where mmap randomization was first introduced to MIPS, the randomization
> range was 256 MiB, equivalent to 16 randomization bits (if 4K page size).
> In 109c32ffd89d ("MIPS: Add support for ARCH_MMAP_RND_{COMPAT_}BITS")
> where MIPS begin to use ARCH_MMAP_RND_BITS, commit message claimed:
> > The minimum(default) for 64bit is 12bits, again with 4k
> > pagesize this is the same as the current 256MB.
> which is incorrect. (1 << 12) * page_size is 256 MiB only when page size
> is 64 KiB, so the strength of mmap randomization was reduced by 4b.
>
> Fixes: 109c32ffd89d ("MIPS: Add support for ARCH_MMAP_RND_{COMPAT_}BITS")
> Signed-off-by: Bingwu Zhang <xtex@xxxxxxxxxxxxx>
> ---
>  arch/mips/Kconfig | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index e48b62b4dc48..c630dab44419 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -3063,11 +3063,11 @@ config MMU
>         default y
>
>  config ARCH_MMAP_RND_BITS_MIN
> -       default 12 if 64BIT
> -       default 8
> -
> -config ARCH_MMAP_RND_BITS_MAX
>         default 18 if 64BIT
> +       default 8
> +
> +config ARCH_MMAP_RND_BITS_MAX
> +       default 20 if 64BIT
>         default 15
>
>  config ARCH_MMAP_RND_COMPAT_BITS_MIN
>
> base-commit: be762d8b6dd7efacb61937d20f8475db8f207655
> --
> 2.52.0
>
>