From: ebiederm@xmission.com (Eric W. Biederman)
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Daniel Cashman <dcashman@android.com>,
linux-kernel@vger.kernel.org, linux@arm.linux.org.uk,
keescook@chromium.org, mingo@kernel.org,
linux-arm-kernel@lists.infradead.org, corbet@lwn.net,
dzickus@redhat.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, dcashman <dcashman@google.com>
Subject: Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.
Date: Tue, 03 Nov 2015 18:40:31 -0600 [thread overview]
Message-ID: <87k2pyppfk.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <20151103160410.34bbebc805c17d2f41150a19@linux-foundation.org> (Andrew Morton's message of "Tue, 3 Nov 2015 16:04:10 -0800")
Andrew Morton <akpm@linux-foundation.org> writes:
> On Tue, 3 Nov 2015 10:10:03 -0800 Daniel Cashman <dcashman@android.com> wrote:
>
>> 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.
>
> Can we please include a very good description of the motivation for this
> change? What is inadequate about the current code, what value does the
> enhancement have to our users, what real-world problems are being solved,
> etc.
>
> Because all we have at present is "greater ASLR protection", which doesn't
> really tell anyone anything.
The description seemed clear to me.
More random bits, more entropy, more work needed to brute force.
8 bits only requires 256 tries (or a 1 in 256) chance to brute force
something.
We have seen in the last couple of months on Android how only having 8 bits
doesn't help much.
Each additional bit doubles the protection (and unfortunately also
increases fragmentation of the userspace address space).
Eric
next prev parent reply other threads:[~2015-11-04 0:49 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-03 18:10 [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR Daniel Cashman
2015-11-03 18:10 ` [PATCH v2 2/2] arm: mm: support ARCH_MMAP_RND_BITS Daniel Cashman
2015-11-03 19:19 ` Kees Cook
2015-11-03 22:39 ` Russell King - ARM Linux
2015-11-03 23:18 ` Kees Cook
2015-11-04 18:22 ` Daniel Cashman
2015-11-03 23:14 ` Daniel Cashman
2015-11-03 23:21 ` Kees Cook
2015-11-04 18:30 ` Daniel Cashman
2015-11-05 18:44 ` Daniel Cashman
2015-11-06 20:52 ` Kees Cook
2015-11-09 3:47 ` Michael Ellerman
2015-11-09 18:56 ` Daniel Cashman
2015-11-09 21:27 ` Kees Cook
2015-11-03 19:16 ` [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR Kees Cook
2015-11-04 0:04 ` Andrew Morton
2015-11-04 0:40 ` Eric W. Biederman [this message]
2015-11-04 1:31 ` Andrew Morton
2015-11-04 19:31 ` Daniel Cashman
2015-11-04 22:00 ` Andrew Morton
2015-11-04 22:10 ` Eric W. Biederman
2015-11-04 22:37 ` Kees Cook
2015-11-04 9:39 ` Michal Hocko
2015-11-04 19:21 ` Eric W. Biederman
2015-11-04 19:36 ` Daniel Cashman
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