On 11/11/2013 11:37 AM, Kees Cook wrote: > On Mon, Nov 11, 2013 at 11:27 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote: >> On 11/11/2013 10:20 AM, Ingo Molnar wrote: >>> >>> Why aren't the 3 sources of entropy XOR-ed together? >> >> Note we don't want to poke i8254 if we have any other sources, as the >> i8254 may not be present on newer systems and in some other cases not >> safe to touch. We can XOR in the TSC with RDRAND although it seems a >> bit odd. > > I await the final decision! :) > Use the i8254 if and only if nothing else is available, it isn't safe to touch on some newer systems. >> HOWEVER... >> >> + /* Only use the low bits of rdtsc. */ >> + random = raw & 0xffff; >> >> Why?! All that can do is throw away randomness... > > Due to the 2GiB addressing and 2MiB alignment, we can never use more > than 10 bits of entropy at maximum. The existing patches only use 9 > bits (due to page table layouts) on 64-bit and 8 bits on 32-bit. The point is that you are already doing that masking somewhere else, it is just odd. >> + /* Extend timer bits poorly... */ >> + random |= (random << 16); >> +#ifdef CONFIG_X86_64 >> + random |= (random << 32); >> +#endif > > I did this so that any portion of the returned value could be used for > the 10 bit mask. Why? It is standard practice to only use the low bits, and in light of the above it is even more bizarre. However, doing a circular multiply might still spread in some additional entropy at least in the case of a kexec boot on a non-RDRAND system. >> For smearing out bits, a better way is usually to multiply with a large >> prime number (preferably a circular multiply.) > > This shouldn't be needed since we're only using the low 10 bits. None of the above is needed in that case, although a spreading multiply will make it less likely that you are throwing away the highest entropy bits. > How would you like to see this function updated to make that more > clear? More comments? -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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