On 05/18/2016 03:28 PM, Hannes Frederic Sowa wrote:
On 18.05.2016 15:20, Eric Dumazet wrote:
On Wed, 2016-05-18 at 14:14 +0200, Daniel Borkmann wrote:
Start address randomization and blinding in BPF currently use
prandom_u32(). prandom_u32() values are not exposed to unpriviledged
user space to my knowledge, but given other kernel facilities such as
ASLR, stack canaries, etc make use of stronger get_random_int(), we
better make use of it here as well given blinding requests successively
new random values. get_random_int() has minimal entropy pool depletion,
is not cryptographically secure, but doesn't need to be for our use
cases here.
Well, if it is not crypto secure, what is the point using it instead of
prandom_u32() ?
Leaks of prandom_u32 allows an attacker to find the whole sequence of
generated random numbers or the other way around.
I do not think changing this is fundamentally changing something, it
looks like code churn or magic incantation to me.
I saw this during the review of the blinding patches. I was afraid, that
by extracting or dumping the ebpf program, a user could find out the
blinding constant and after some retries could find out the coefficients
to the taus rng, thus being able to infer the whole sequence generated
by prandom_u32.
Note that dumping/extraction is normally not possible: cBPF progs hold the
original insns before they were even transformed into eBPF, and eBPF cannot
be dumped back today, so the rewrites stay kernel internal. Only when an
admin is not careful and accidentally sets things like bpf_jit_enable > 1,
where we have JIT debug mode and the generated image goes to dmesg (I don't
consider this the case normally, but what do I know).
There is little amount of entropy since the hole is constrained by
PAGE_SIZE.
True.
I don't consider this a big thing, I just mentioned that we probably
shouldn't use prandom_u32 if the value somehow could leak to user space
and should be used for security.
Agree here which is why I decided to send it.
Thanks,
Daniel
