Re: [PATCH] avoid entropy starvation due to stack protection
From: Stephan MÃller
Date: Sat Dec 15 2012 - 17:59:56 EST
Am 15.12.2012 20:15, schrieb OndÅej BÃlka:
Why not use nonblocking pool and seed nonblocking pool only with half of
collected entropy to get /dev/random in almost all practical scenarios
nonblocking?
I would not recommend changing /dev/urandom. First, we would change the
characteristic of a kernel interface a lot of user space cryptographic
components rely on. According to Linus that is typically a no-go.
Moreover, the question can be raised, where do we pick the number of
50%, why not 30% or 70%, why (re)seeding it at all?
Also, let us assume we pick 50% and we leave the create_elf_tables
function as is (i.e. it pulls from get_random_bytes), I fear that we do
not win at all. Our discussed problem is the depletion of the entropy
via nonblocking_pool due to every execve() syscall requires 128 bits of
data from nonblocking_pool. Even if we seed nonblocking_pool more
rarely, we still deplete the entropy of the input_pool and thus deplete
the entropy we want for cryptographic purposes a particular user has.
Thus, my recommendation is to disconnect the system entropy requirements
from the user entropy requirements as much as possible. I am aware that
there are in-kernel cryptographic requirements that must seed itself via
the good entropy. And those users shall be rather left untouched -- i.e.
they should still call get_random_bytes.
But for users that do not require cryptographic strength, but a strength
against guessing of a random number on the local system for a decent
time (like the stack protection or ASLR), we can use a slightly less
perfect DRNG which is seeded with good entropy and never thereafter.
Ciao
Stephan
On Thu, Dec 13, 2012 at 08:44:36AM +0100, Stephan Mueller wrote:
On 13.12.2012 01:43:21, +0100, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
Hi Andrew,
On Tue, 11 Dec 2012 13:33:04 +0100
Stephan Mueller<smueller@xxxxxxxxxx> wrote:
Some time ago, I noticed the fact that for every newly
executed process, the function create_elf_tables requests 16 bytes of
randomness from get_random_bytes. This is easily visible when calling
while [ 1 ]
do
cat /proc/sys/kernel/random/entropy_avail
sleep 1
done
Please see
http://ozlabs.org/~akpm/mmotm/broken-out/binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patch
That patch is about one week from a mainline merge, btw.
Initially I was also thinking about get_random_int. But stack protection
depends on non-predictable numbers to ensure it cannot be defeated. As
get_random_int depends on MD5 which is assumed to be broken now, I
discarded the idea of using get_random_int.
Moreover, please consider that get_cycles is an architecture-specific
function that on some architectures only returns 0 (For all
architectures where this is implemented, you have no guarantee that it
increments as a high-resolution timer). So, the quality of
get_random_int is questionable IMHO for the use as a stack protector.
Also note, that other in-kernel users of get_random_bytes may be
converted to using the proposed kernel pool to avoid more entropy drainage.
Please note that the suggested approach of fully seeding a deterministic
RNG never followed by a re-seeding is used elsewhere (e.g. the OpenSSL
RNG). Therefore, I think the suggested approach is viable.
Ciao
Stephan
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