Re: [PATCH 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.

[ebiederm@xmission.com (Eric W. Biederman)]

Daniel Cashman <dcashman@android.com> writes:

> From: dcashman <dcashman@google.com>
>
> ASLR currently only uses 8 bits to generate the random offset for the
> mmap base address on 32 bit architectures. This value was chosen to
> prevent a poorly chosen value from dividing the address space in such
> a way as to prevent large allocations. This may not be an issue on all
> platforms. Allow the specification of a minimum number of bits so that
> platforms desiring greater ASLR protection may determine where to place
> the trade-off.

This all would be much cleaner if the arm architecture code were just to
register the sysctl itself.

As it sits this looks like a patchset that does not meaninfully bisect,
and would result in code that is hard to trace and understand.

Eric

> Signed-off-by: Daniel Cashman <dcashman@google.com>
> ---
>  Documentation/sysctl/kernel.txt | 14 ++++++++++++++
>  include/linux/mm.h              |  6 ++++++
>  kernel/sysctl.c                 | 11 +++++++++++
>  3 files changed, 31 insertions(+)
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 6fccb69..0d4ca53 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -41,6 +41,7 @@ show up in /proc/sys/kernel:
>  - kptr_restrict
>  - kstack_depth_to_print       [ X86 only ]
>  - l2cr                        [ PPC only ]
> +- mmap_rnd_bits
>  - modprobe                    ==> Documentation/debugging-modules.txt
>  - modules_disabled
>  - msg_next_id		      [ sysv ipc ]
> @@ -391,6 +392,19 @@ This flag controls the L2 cache of G3 processor boards. If
>  
>  ==============================================================
>  
> +mmap_rnd_bits:
> +
> +This value can be used to select the number of bits to use to
> +determine the random offset to the base address of vma regions
> +resulting from mmap allocations on architectures which support
> +tuning address space randomization.  This value will be bounded
> +by the architecture's minimum and maximum supported values.
> +
> +This value can be changed after boot using the
> +/proc/sys/kernel/mmap_rnd_bits tunable
> +
> +==============================================================
> +
>  modules_disabled:
>  
>  A toggle value indicating if modules are allowed to be loaded
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 80001de..15b083a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -51,6 +51,12 @@ extern int sysctl_legacy_va_layout;
>  #define sysctl_legacy_va_layout 0
>  #endif
>  
> +#ifdef CONFIG_ARCH_MMAP_RND_BITS
> +extern int mmap_rnd_bits_min;
> +extern int mmap_rnd_bits_max;
> +extern int mmap_rnd_bits;
> +#endif
> +
>  #include <asm/page.h>
>  #include <asm/pgtable.h>
>  #include <asm/processor.h>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index e69201d..37e657a 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1139,6 +1139,17 @@ static struct ctl_table kern_table[] = {
>  		.proc_handler	= timer_migration_handler,
>  	},
>  #endif
> +#ifdef CONFIG_ARCH_MMAP_RND_BITS
> +	{
> +		.procname	= "mmap_rnd_bits",
> +		.data		= &mmap_rnd_bits,
> +		.maxlen		= sizeof(mmap_rnd_bits),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &mmap_rnd_bits_min,
> +		.extra2		= &mmap_rnd_bits_max,
> +	},
> +#endif
>  	{ }
>  };

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