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

[Kees Cook <kees@kernel.org>]

On Sun, Mar 29, 2026 at 06:57:39AM +0800, Bingwu Zhang wrote:
> From: Bingwu Zhang <xtex@astrafall.org>
> 
> 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.
> 
> 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@astrafall.org>

I defer to MIPS maintainers, but yeah, let's turn it up if possible.

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook