Messages in this thread | | | Date | Fri, 21 Feb 2014 13:21:54 -0800 | Subject | Re: [PATCH] x86, kaslr: randomize module base load address | From | Kees Cook <> |
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On Fri, Feb 21, 2014 at 1:15 PM, Andrew Morton <akpm@linux-foundation.org> wrote: > On Fri, 21 Feb 2014 13:05:08 -0800 Kees Cook <keescook@chromium.org> wrote: > >> >> +#ifdef CONFIG_RANDOMIZE_BASE >> >> +static unsigned long module_load_offset; >> >> +static int randomize_modules = 1; >> > >> > It's pretty common for people to later come back and say "hey I want to >> > set the default in Kconfig". Perhaps we should do that from day 1. >> >> I've been slapped down for adding more config options in the past, and >> I think it's unlikely that people using CONFIG_RANDOMIZE_BASE won't >> want the modules base randomized too. I think this is a safe default, >> but if you see it as a requirement, I can change it. > > I think there were issues with some embedded systems where it's > hard/impossible to provide/alter boot parameters. > > I suppose we can leave it this way until there are complaints. > >> > This implies that parse_nokaslr() will need to be renamed and taught to >> > handle 0->1 changing. >> > >> >> +static int __init parse_nokaslr(char *p) >> >> +{ >> >> + randomize_modules = 0; >> >> + return 0; >> >> +} >> >> +early_param("nokaslr", parse_nokaslr); >> > >> > Documentation/kernel-parameters.txt, please. This isn't hard :( >> >> "nokaslr" is already documented. Do you mean adding a note about >> modules as well to the existing documentation? > > Yes, it should now mention modules as well.
Okay, I will add this to the v2 patch.
> >> >> +static unsigned long int get_module_load_offset(void) >> >> +{ >> >> + if (randomize_modules) { >> >> + mutex_lock(&module_mutex); >> >> + /* >> >> + * Calculate the module_load_offset the first time this >> >> + * code is called. Once calculated it stays the same until >> >> + * reboot. >> >> + */ >> >> + if (module_load_offset == 0) >> >> + module_load_offset = >> >> + (get_random_int() % 1024 + 1) * PAGE_SIZE; >> >> + mutex_unlock(&module_mutex); >> >> + } >> >> + return module_load_offset; >> >> +} >> > >> > This seems unnecessarily complex and inefficient. We only set >> > module_load_offset a single time, so why not do it that way? >> > Mark it __init, run it during initcalls then throw it away. >> >> I'd like to make sure this is running well after the pRNG is up and >> running. I can run some tests to see how the entropy looks if this is >> done during __init, though. > > That may be a bit optimistic, dunno. I suppose that doing it this way > we will already have done a bit of disk IO, so there will be more > randomness.
Right -- it's probably not much, but this doesn't seem like much overhead for module loading. It's a relatively infrequent event.
> btw, would it be better to make each module have its own offset rather > than using the same offset for all of them? That could cause problems > with vmap space fragmentation I guess.
Right -- we felt this was sufficient. And in my experience, other attempts and trying to do per-allocat randomization and avoid the fragmentation problem (e.g. "greedy random positioning"), like done with RedHat's ASCII-Armor memory randomization proved to have _less_ entropy than just randomizing the base address.
-Kees
-- Kees Cook Chrome OS Security
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