Subject: Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base
 ASLR.
To: Michael Ellerman <mpe@ellerman.id.au>, linux-kernel@vger.kernel.org
References: <1447888808-31571-1-git-send-email-dcashman@android.com>
 <1447888808-31571-2-git-send-email-dcashman@android.com>
 <1448426400.3762.11.camel@ellerman.id.au>
Cc: linux@arm.linux.org.uk, akpm@linux-foundation.org, keescook@chromium.org,
        mingo@kernel.org, linux-arm-kernel@lists.infradead.org, corbet@lwn.net,
        dzickus@redhat.com, ebiederm@xmission.com, xypron.glpk@gmx.de,
        jpoimboe@redhat.com, kirill.shutemov@linux.intel.com,
        n-horiguchi@ah.jp.nec.com, aarcange@redhat.com, mgorman@suse.de,
        tglx@linutronix.de, rientjes@google.com, linux-mm@kvack.org,
        linux-doc@vger.kernel.org, salyzyn@android.com, jeffv@google.com,
        nnk@google.com, catalin.marinas@arm.com, will.deacon@arm.com,
        hpa@zytor.com, x86@kernel.org, hecmargi@upv.es, bp@suse.de,
        dcashman@google.com
From: Daniel Cashman <dcashman@android.com>
Message-ID: <56560DCA.7050009@android.com>
Date: Wed, 25 Nov 2015 11:36:42 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.3.0
MIME-Version: 1.0
In-Reply-To: <1448426400.3762.11.camel@ellerman.id.au>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2541
Lines: 77

On 11/24/2015 08:40 PM, Michael Ellerman wrote:
> On Wed, 2015-11-18 at 15:20 -0800, Daniel Cashman wrote:
> 
>> 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.
> 
> ...
> 
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 4e949e5..141823f 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -511,6 +511,70 @@ config ARCH_HAS_ELF_RANDOMIZE
>>  	  - arch_mmap_rnd()
>>  	  - arch_randomize_brk()
>>  
>> +config HAVE_ARCH_MMAP_RND_BITS
>> +	bool
>> +	help
>> +	  An arch should select this symbol if it supports setting a variable
>> +	  number of bits for use in establishing the base address for mmap
>> +	  allocations and provides values for both:
>> +	  - ARCH_MMAP_RND_BITS_MIN
>> +	  - ARCH_MMAP_RND_BITS_MAX
>> +
>> +config ARCH_MMAP_RND_BITS_MIN
>> +	int
>> +
>> +config ARCH_MMAP_RND_BITS_MAX
>> +	int
>> +
>> +config ARCH_MMAP_RND_BITS_DEFAULT
>> +	int
>> +
>> +config ARCH_MMAP_RND_BITS
>> +	int "Number of bits to use for ASLR of mmap base address" if EXPERT
>> +	range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
>> +	default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
> 
> Here you support a default which is separate from the minimum.
> 
>> +	default ARCH_MMAP_RND_BITS_MIN
>> +	depends on HAVE_ARCH_MMAP_RND_BITS
> 
> ...
>> +
>> +config ARCH_MMAP_RND_COMPAT_BITS
>> +	int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
>> +	range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
>> +	default ARCH_MMAP_RND_COMPAT_BITS_MIN
> 
> But here you don't.
> 
> Just forgot?

Yes.  Good catch.

> I'd like to have a default which is separate from the minimum. That way we can
> have a default which is reasonably large, but allow it to be lowered easily if
> anything breaks.

Will add it, along w/the documentation cleanup and other changes.

Thank You,
Dan
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/