linux-kernel - Re: [PATCH 0/2] Fix mmap_base entropy for >31 bits.

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Date:	Wed, 24 Feb 2016 13:50:47 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Daniel Cashman <dcashman@...roid.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>, Arnd Bergmann <arnd@...db.de>,
	Greg KH <gregkh@...uxfoundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	"David S. Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Nick Kralevich <nnk@...gle.com>,
	Jeffrey Vander Stoep <jeffv@...gle.com>,
	Mark Salyzyn <salyzyn@...roid.com>
Subject: Re: [PATCH 0/2] Fix mmap_base entropy for >31 bits.

On Wed, Feb 24, 2016 at 12:40 PM, Daniel Cashman <dcashman@...roid.com> wrote:
> On 02/04/2016 02:29 PM, Kees Cook wrote:
>> On Thu, Feb 4, 2016 at 2:06 PM, Daniel Cashman <dcashman@...roid.com> wrote:
>>> Upstream commit: d07e22597d1d355829b7b18ac19afa912cf758d1 added the
>>> ability to choose from a range of values to use for entropy count in
>>> generating the random offset to the mmap_base address.  The
>>> maximum value on this range was set to 32 bits for 64-bit x86 systems,
>>> but this value could be increased further, requiring more than the 32
>>> bits of randomness provided by get_random_int(), as is already possible
>>> for arm64. Add a new function: get_random_long() which more naturally
>>> fits with the mmap usage of get_random_int() but operates exactly the
>>> same as get_random_int().
>>>
>>> Also, fix the shifting constant in mmap_rnd() to be an unsigned long so
>>> that values greater than 31 bits generate an appropriate mask without
>>> overflow.  This is especially important on x86, as its shift instruction
>>> uses a 5-bit mask for the shift operand, which meant that any value for
>>> mmap_rnd_bits over 31 acts as a no-op and effectively disables mmap_base
>>> randomization.
>>>
>>> Finally, replace calls to get_random_int() with get_random_long() where
>>> appropriate.
>>>
>>> Daniel Cashman (2):
>>>   drivers: char: random: Add get_random_long().
>>>   use get_random_long().
>>>
>>>  arch/arm/mm/mmap.c               |  2 +-
>>>  arch/arm64/mm/mmap.c             |  4 ++--
>>>  arch/mips/mm/mmap.c              |  4 ++--
>>>  arch/powerpc/kernel/process.c    |  4 ++--
>>>  arch/powerpc/mm/mmap.c           |  4 ++--
>>>  arch/sparc/kernel/sys_sparc_64.c |  2 +-
>>>  arch/x86/mm/mmap.c               |  6 +++---
>>>  drivers/char/random.c            | 22 ++++++++++++++++++++++
>>>  fs/binfmt_elf.c                  |  2 +-
>>>  include/linux/random.h           |  1 +
>>>  10 files changed, 37 insertions(+), 14 deletions(-)
>>
>> Thanks for fixing this!
>>
>> Acked-by: Kees Cook <keescook@...omium.org>
>>
>> -Kees
>>
>
> This has now been in linux-next for awhile. Could we please submit this
> for the 4.5 rc window so that it fixes the issue in the final 4.5
> release?  Sorry if this is a protocol breach, but I'm not sure what the
> best way is for me to indicate that this is a "fix" that should go out
> in the same release as the original feature.

Yes please! :) Andrew, can you push these for 4.5? Best to keep the
feature from releasing broken.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security