Messages in this thread | | | Date | Sat, 15 Dec 2012 23:59:46 +0100 | From | Stephan Müller <> | Subject | Re: [PATCH] avoid entropy starvation due to stack protection |
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Am 15.12.2012 20:15, schrieb Ondřej Bílka: > Why not use nonblocking pool and seed nonblocking pool only with half of > collected entropy to get /dev/random in almost all practical scenarios > nonblocking?
I would not recommend changing /dev/urandom. First, we would change the characteristic of a kernel interface a lot of user space cryptographic components rely on. According to Linus that is typically a no-go. Moreover, the question can be raised, where do we pick the number of 50%, why not 30% or 70%, why (re)seeding it at all?
Also, let us assume we pick 50% and we leave the create_elf_tables function as is (i.e. it pulls from get_random_bytes), I fear that we do not win at all. Our discussed problem is the depletion of the entropy via nonblocking_pool due to every execve() syscall requires 128 bits of data from nonblocking_pool. Even if we seed nonblocking_pool more rarely, we still deplete the entropy of the input_pool and thus deplete the entropy we want for cryptographic purposes a particular user has.
Thus, my recommendation is to disconnect the system entropy requirements from the user entropy requirements as much as possible. I am aware that there are in-kernel cryptographic requirements that must seed itself via the good entropy. And those users shall be rather left untouched -- i.e. they should still call get_random_bytes.
But for users that do not require cryptographic strength, but a strength against guessing of a random number on the local system for a decent time (like the stack protection or ASLR), we can use a slightly less perfect DRNG which is seeded with good entropy and never thereafter.
Ciao Stephan > > On Thu, Dec 13, 2012 at 08:44:36AM +0100, Stephan Mueller wrote: >> On 13.12.2012 01:43:21, +0100, Andrew Morton >> <akpm@linux-foundation.org> wrote: >> >> Hi Andrew, >>> On Tue, 11 Dec 2012 13:33:04 +0100 >>> Stephan Mueller<smueller@chronox.de> wrote: >>> >>>> Some time ago, I noticed the fact that for every newly >>>> executed process, the function create_elf_tables requests 16 bytes of >>>> randomness from get_random_bytes. This is easily visible when calling >>>> >>>> while [ 1 ] >>>> do >>>> cat /proc/sys/kernel/random/entropy_avail >>>> sleep 1 >>>> done >>> Please see >>> http://ozlabs.org/~akpm/mmotm/broken-out/binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patch >>> >>> That patch is about one week from a mainline merge, btw. >> Initially I was also thinking about get_random_int. But stack protection >> depends on non-predictable numbers to ensure it cannot be defeated. As >> get_random_int depends on MD5 which is assumed to be broken now, I >> discarded the idea of using get_random_int. >> >> Moreover, please consider that get_cycles is an architecture-specific >> function that on some architectures only returns 0 (For all >> architectures where this is implemented, you have no guarantee that it >> increments as a high-resolution timer). So, the quality of >> get_random_int is questionable IMHO for the use as a stack protector. >> >> Also note, that other in-kernel users of get_random_bytes may be >> converted to using the proposed kernel pool to avoid more entropy drainage. >> >> Please note that the suggested approach of fully seeding a deterministic >> RNG never followed by a re-seeding is used elsewhere (e.g. the OpenSSL >> RNG). Therefore, I think the suggested approach is viable. >> >> Ciao >> Stephan >> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/
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