[PATCH] arm64: raise default ARCH_MMAP_RND_BITS for 48-bit and 52-bit VA

[PATCH] arm64: raise default ARCH_MMAP_RND_BITS for 48-bit and 52-bit VA

[Iurii Trukhin]

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
Signed-off-by: Iurii Trukhin <yuri@trukhin.com>
---
 arch/arm64/Kconfig | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index cab741a695f5..59a53d7a75c5 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -313,6 +313,11 @@ config ARCH_MMAP_RND_BITS_MAX
 	default 16 if ARM64_16K_PAGES
 	default 18

+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

Re: [PATCH] arm64: raise default ARCH_MMAP_RND_BITS for 48-bit and 52-bit VA

[Kees Cook]

On Wed, Oct 14, 2026 at 07:26:48PM -0500, 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
> Signed-off-by: Iurii Trukhin <yuri@trukhin.com>

Yes please. :)

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

-- 
Kees Cook