On Wed, Oct 14, 2026 at 10:23:35PM -0700, Iurii Trukhin wrote:
> Since commit f101c5644771 ("arm64: Support ARM64_VA_BITS=52 when
> setting ARCH_MMAP_RND_BITS_MAX"), arm64 supports up to 33, 31, and 29
> bits of mmap ASLR entropy on 48-bit and 52-bit virtual address spaces
> for 4K, 16K, and 64K pages respectively. However, ARCH_MMAP_RND_BITS
> still defaults to ARCH_MMAP_RND_BITS_MIN (18, 16, and 14 bits), which
> was set in commit 8f0d3aa9de57 ("arm64: mm: support ARCH_MMAP_RND_BITS")
> when 39-bit VA was the 4K default.
>
> Set the default ARCH_MMAP_RND_BITS for 48-bit and 52-bit VA to 28 (4K
> pages), 26 (16K pages), and 24 (64K pages), matching x86_64's 1 TiB
> randomization range (5 bits below ARCH_MMAP_RND_BITS_MAX) while keeping
> ARCH_MMAP_RND_BITS_MIN unchanged for smaller VA configurations.
>
> Closes: https://github.com/KSPP/linux/issues/371
> Reviewed-by: Kees Cook <[email protected]>
> Signed-off-by: Iurii Trukhin <[email protected]>
Reviewed-by: Anshuman Khandual <[email protected]>
> ---
> Changes in v2:
> - Add a comment above ARCH_MMAP_RND_BITS explaining the 1 TiB default
> for 48/52-bit VA and the fallback to ARCH_MMAP_RND_BITS_MIN in
> arch/Kconfig for smaller VA sizes (Anshuman Khandual).
> - Collect Reviewed-by from Kees Cook.
> - Link to v1:
> https://lore.kernel.org/r/CAOx7e9Jm0N9O_ndL1UApMPrXUN=p4riptsnibddp+9prusn...@mail.gmail.com
> arch/arm64/Kconfig | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index cab741a695f5..7bca47269dd5 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -313,6 +313,14 @@ config ARCH_MMAP_RND_BITS_MAX
> default 16 if ARM64_16K_PAGES
> default 18
>
> +# default to 1 TiB (5 bits below ARCH_MMAP_RND_BITS_MAX) on 48-bit and
> +# 52-bit VA; smaller VA sizes fall back to ARCH_MMAP_RND_BITS_MIN in
> +# arch/Kconfig
> +config ARCH_MMAP_RND_BITS
> + default 24 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
> + default 26 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
> + default 28 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
> +
> config ARCH_MMAP_RND_COMPAT_BITS_MIN
> default 7 if ARM64_64K_PAGES
> default 9 if ARM64_16K_PAGES
> --
> 2.56.0
