Hi!
> > x86, kaslr: Provide randomness functions
> >
> > Adds potential sources of randomness: RDRAND, RDTSC, or the i8254.
> >
> > This moves the pre-alternatives inline rdrand function into the header so
> > both pieces of code can use it. Availability of RDRAND is then controlled
> > by CONFIG_ARCH_RANDOM, if someone wants to disable it even for kASLR.
>
> While reviewing this as a pre-pull-request, I noticed the following
> detail:
>
> > +static unsigned long get_random_long(void)
> > +{
> > + unsigned long random;
> > +
> > + if (has_cpuflag(X86_FEATURE_RDRAND)) {
> > + debug_putstr("KASLR using RDRAND...\n");
> > + if (rdrand_long(&random))
> > + return random;
> > + }
> > +
> > + if (has_cpuflag(X86_FEATURE_TSC)) {
> > + uint32_t raw;
> > +
> > + debug_putstr("KASLR using RDTSC...\n");
> > + rdtscl(raw);
> > +
> > + /* Only use the low bits of rdtsc. */
> > + random = raw & 0xffff;
> > + } else {
> > + debug_putstr("KASLR using i8254...\n");
> > + random = i8254();
> > + }
> > +
> > + /* Extend timer bits poorly... */
> > + random |= (random << 16);
> > +#ifdef CONFIG_X86_64
> > + random |= (random << 32);
> > +#endif
> > + return random;
> > +}
>
> Why aren't the 3 sources of entropy XOR-ed together?
>
> Also, we talked about also adding system dependent entropy sources, such
> as memory layout or the DMI table - none of that seems to have
> happened.
> It's not like this function should be performance critical, it's run once
> per bootup, right? There's just no excuse for not maximizing available
> entropy in such a situation ...
If it is not performance critical, why not use "/dev/random"
infrastructure?
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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