Messages in this thread | | | From | "PaX Team" <> | Date | Tue, 07 Jun 2016 14:19:14 +0200 | Subject | Re: [kernel-hardening] Re: [PATCH v2 1/3] Add the latent_entropy gcc plugin |
| |
On 6 Jun 2016 at 19:13, Theodore Ts'o wrote:
> On Mon, Jun 06, 2016 at 09:30:12PM +0200, PaX Team wrote: > > > > what matters for latent entropy is not the actual values fed into the entropy > > pool (they're effectively compile time constants save for runtime data dependent > > computations) but the precise sequence of them. interrupts stir this sequence > > and thus extract entropy. perhaps as a small example imagine that an uninterrupted > > kernel boot sequence feeds these values into the entropy pool: > > A B C > > > > now imagine that a single interrupt can occur around any one of these values: > > I A B C > > A I B C > > A B I C > > A B C I > > > > this way we can obtain 4 different final pool states that translate into up > > to 2 bits of latent entropy (depends on how probable each sequence is). note > > that this works regardless whether the underlying hardware has a high resolution > > timer whose values the interrupt handler would feed into the pool. > > Right, but if it's only about interrupts,
(i believe that) latent entropy is found in more than just interrupt timing, there're also data dependent computations that can have entropy, either on a single system or across a population of them.
> we're doing this already inside modern Linux kernels. On every single > interrupt we are mixing into a per-CPU "fast mix" pool the IP from the > interrupt registers.
i agree that sampling the kernel register state can have entropy (the plugin already extracts the current stack pointer) but i'm much less sure about userland (at least i see no dependence on !user_mode(...)) since an attacker could feed no entropy into the pool but still get it credited.
cheers, PaX Team
| |