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Date: Tue, 10 Dec 2013 12:27:45 -0800 From: Kees Cook <keescook@...omium.org> To: Ingo Molnar <mingo@...hat.com> Cc: linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org Subject: [PATCH -next] x86, kaslr: clarify RANDOMIZE_BASE_MAX_OFFSET The help text for RANDOMIZE_BASE_MAX_OFFSET was confusing. This has been clarified, and updated to be an export-only tunable. Signed-off-by: Kees Cook <keescook@...omium.org> --- arch/x86/Kconfig | 29 ++++++++++++++++++----------- 1 file changed, 18 insertions(+), 11 deletions(-) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index a5c83b9dad45..e5fd1e67fd73 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -1752,26 +1752,33 @@ config RANDOMIZE_BASE possible. At best, due to page table layouts, 64-bit can use 9 bits of entropy and 32-bit uses 8 bits. + If unsure, say N. + config RANDOMIZE_BASE_MAX_OFFSET - hex "Maximum ASLR offset allowed" + hex "Maximum kASLR offset allowed" if EXPERT depends on RANDOMIZE_BASE range 0x0 0x20000000 if X86_32 default "0x20000000" if X86_32 range 0x0 0x40000000 if X86_64 default "0x40000000" if X86_64 ---help--- - Determines the maximal offset in bytes that will be applied to the - kernel when Address Space Layout Randomization (ASLR) is active. - Must be less than or equal to the actual physical memory on the - system. This must be a multiple of CONFIG_PHYSICAL_ALIGN. + The lesser of RANDOMIZE_BASE_MAX_OFFSET and available physical + memory is used to determine the maximal offset in bytes that will + be applied to the kernel when kernel Address Space Layout + Randomization (kASLR) is active. This must be a multiple of + PHYSICAL_ALIGN. + + On 32-bit this is limited to 512MiB by page table layouts. The + default is 512MiB. - On 32-bit this is limited to 512MiB. + On 64-bit this is limited by how the kernel fixmap page table is + positioned, so this cannot be larger than 1GiB currently. Without + RANDOMIZE_BASE, there is a 512MiB to 1.5GiB split between kernel + and modules. When RANDOMIZE_BASE_MAX_OFFSET is above 512MiB, the + modules area will shrink to compensate, up to the current maximum + 1GiB to 1GiB split. The default is 1GiB. - On 64-bit this is limited by how the kernel fixmap page table is - positioned, so this cannot be larger that 1GiB currently. Normally - there is a 512MiB to 1.5GiB split between kernel and modules. When - this is raised above the 512MiB default, the modules area will - shrink to compensate, up to the current maximum 1GiB to 1GiB split. + If unsure, leave at the default value. # Relocation on x86 needs some additional build support config X86_NEED_RELOCS -- 1.7.9.5 -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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