On Wed, Mar 5, 2014 at 4:52 PM, Matt Mackall <mpm@xxxxxxxxxxx> wrote: > On Wed, 2014-03-05 at 16:11 -0500, Jason Cooper wrote: >> > In other words, if there are 4096 bits of "unknownness" in X to start >> > with, and I can get those same 4096 bits of "unknownness" back by >> > unmixing X' and Y, then there must still be 4096 bits of "unknownness" >> > in X'. If X' is 4096 bits long, then we've just proven that >> > reversibility means the attacker can know nothing about the contents of >> > X' by his choice of Y. >> >> Well, this reinforces my comfortability with loadable modules. The pool >> is already initialized by the point at which the driver is loaded. >> >> Unfortunately, any of the drivers in hw_random can be built in. When >> built in, hwrng_register is going to be called during the kernel >> initialization process. In that case, the unknownness in X is not 4096 >> bits, but far less. Also, the items that may have seeded X (MAC addr, >> time, etc) are discoverable by a potential attacker. This is also well >> before random-seed has been fed in. > > To which I would respond.. so? > > If the pool is in an attacker-knowable state at early boot, adding > attacker-controlled data does not make the situation any worse. In fact, > if the attacker has less-than-perfect control of the inputs, mixing more > things in will make things exponentially harder for the attacker. > > Put another way: mixing can't ever removes unknownness from the pool, it > can only add more. So the only reason you should ever choose not to mix > something into the pool is performance. Excellent. So it sounds like you're okay with my original patch as-is? -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
