linux-kernel - Re: [PATCH v4 4/4] x86: mm: support ARCH_MMAP_RND

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Date:	Tue, 1 Dec 2015 10:19:32 -0800
From:	Daniel Cashman <dcashman@...roid.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Jonathan Corbet <corbet@....net>,
	Don Zickus <dzickus@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Heinrich Schuchardt <xypron.glpk@....de>, jpoimboe@...hat.com,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	n-horiguchi@...jp.nec.com, Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Linux-MM <linux-mm@...ck.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Mark Salyzyn <salyzyn@...roid.com>,
	Jeffrey Vander Stoep <jeffv@...gle.com>,
	Nick Kralevich <nnk@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>, Hector Marco <hecmargi@....es>,
	Borislav Petkov <bp@...e.de>,
	Daniel Cashman <dcashman@...gle.com>
Subject: Re: [PATCH v4 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

On 11/30/2015 04:03 PM, Kees Cook wrote:
> On Thu, Nov 26, 2015 at 2:59 PM, Daniel Cashman <dcashman@...roid.com> wrote:
>> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
>> index 844b06d..647fecf 100644
>> --- a/arch/x86/mm/mmap.c
>> +++ b/arch/x86/mm/mmap.c
>> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
>>  {
>>         unsigned long rnd;
>>
>> -       /*
>> -        *  8 bits of randomness in 32bit mmaps, 20 address space bits
>> -        * 28 bits of randomness in 64bit mmaps, 40 address space bits
>> -        */
>>         if (mmap_is_ia32())
>> -               rnd = (unsigned long)get_random_int() % (1<<8);
>> +#ifdef CONFIG_COMPAT
>> +               rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
>> +#else
>> +               rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>> +#endif
>>         else
>> -               rnd = (unsigned long)get_random_int() % (1<<28);
>> +               rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>>
>>         return rnd << PAGE_SHIFT;
>>  }
>> --
>> 2.6.0.rc2.230.g3dd15c0
>>
> 
> Can you rework this logic to look more like the arm64 one? I think
> it's more readable as:
> 
> #ifdef CONFIG_COMPAT
>     if (mmap_is_ia32())
>             rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
>     else
> #endif
>             rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
> 
> -Kees
> 

There is a subtle difference between the two that requires this
difference. the x86 code was written to be used by both 32-bit and
64-bit kernels, whereas the arm64 code runs only for 64-bit.  The
assumption I've made with arm64 is that TIF_32BIT should never be set if
CONFIG_COMPAT is not set, but with x86 we could encounter a 32-bit
application without CONFIG_COMPAT, in which case it should use the
default mmap_rnd_bits, not compat, since there is no compat.

-Dan
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