On 13.12.2012 01:43:21, +0100, Andrew Morton <[email protected]> wrote:
Hi Andrew, > On Tue, 11 Dec 2012 13:33:04 +0100 > Stephan Mueller <[email protected]> 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 [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
