Messages in this thread | | | Date | Wed, 07 Nov 2012 12:21:27 +0800 | From | Jeff Liu <> | Subject | Re: + binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patch added to -mm tree |
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Hi Andrew and Kees,
Great thanks for both your comments!
On 11/07/2012 09:11 AM, Kees Cook wrote: > Hrm, I don't like this. get_random_int() specifically says: "Get a > random word for internal kernel use only." The intent of AT_RANDOM is > for userspace pRNG seeding (though glibc currently uses it directly > for stack protector and pointer mangling), which is not "internal > kernel use only". :) Though I suppose this is already being used for > the randomize_stack_top(), but I think it'd still be better to use > higher quality bits. Btw Kees, does it sounds make sense if we just return the 16 bytes uninitialized stack array if the user disable the stack randomize via "/proc/sys/kernel/randomize_va_space = 0" or via the related sysctl, or even specified norandmaps on boot?
I guess this sounds more stupid since some scripts kids would like it for writing exploits. :-P > > Notes below... > > On Tue, Nov 6, 2012 at 4:16 PM, <akpm@linux-foundation.org> wrote: >> >> The patch titled >> Subject: binfmt_elf.c: use get_random_int() to fix entropy depleting >> has been added to the -mm tree. Its filename is >> binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patch >> >> Before you just go and hit "reply", please: >> a) Consider who else should be cc'ed >> b) Prefer to cc a suitable mailing list as well >> c) Ideally: find the original patch on the mailing list and do a >> reply-to-all to that, adding suitable additional cc's >> >> *** Remember to use Documentation/SubmitChecklist when testing your code *** >> >> The -mm tree is included into linux-next and is updated >> there every 3-4 working days >> >> ------------------------------------------------------ >> From: Jeff Liu <jeff.liu@oracle.com> >> Subject: binfmt_elf.c: use get_random_int() to fix entropy depleting >> >> Entropy is quickly depleted under normal operations like ls(1), cat(1), >> etc... between 2.6.30 to current mainline, for instance: >> >> $ cat /proc/sys/kernel/random/entropy_avail >> 3428 >> $ cat /proc/sys/kernel/random/entropy_avail >> 2911 >> $cat /proc/sys/kernel/random/entropy_avail >> 2620 >> >> We observed this problem has been occurring since 2.6.30 with >> fs/binfmt_elf.c: create_elf_tables()->get_random_bytes(), introduced by >> f06295b44c296c8f ("ELF: implement AT_RANDOM for glibc PRNG seeding"). >> >> /* >> * Generate 16 random bytes for userspace PRNG seeding. >> */ >> get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes)); >> >> The patch introduces a wrapper around get_random_int() which has lower >> overhead than calling get_random_bytes() directly. >> >> With this patch applied: >> $ cat /proc/sys/kernel/random/entropy_avail >> 2731 >> $ cat /proc/sys/kernel/random/entropy_avail >> 2802 >> $ cat /proc/sys/kernel/random/entropy_avail >> 2878 >> >> Analyzed by John Sobecki. >> >> Signed-off-by: Jie Liu <jeff.liu@oracle.com> >> Cc: John Sobecki <john.sobecki@oracle.com> >> Cc: Al Viro <viro@zeniv.linux.org.uk> >> Cc: Andreas Dilger <aedilger@gmail.com> >> Cc: Alan Cox <alan@linux.intel.com> >> Cc: Arnd Bergmann <arnn@arndb.de> >> Cc: James Morris <james.l.morris@oracle.com> >> Cc: Ted Ts'o <tytso@mit.edu> >> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> >> Cc: Kees Cook <keescook@chromium.org> >> Cc: Jakub Jelinek <jakub@redhat.com> >> Cc: Ulrich Drepper <drepper@redhat.com> >> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> >> --- >> >> fs/binfmt_elf.c | 26 +++++++++++++++++++++++++- >> 1 file changed, 25 insertions(+), 1 deletion(-) >> >> diff -puN fs/binfmt_elf.c~binfmt_elfc-use-get_random_int-to-fix-entropy-depleting fs/binfmt_elf.c >> --- a/fs/binfmt_elf.c~binfmt_elfc-use-get_random_int-to-fix-entropy-depleting >> +++ a/fs/binfmt_elf.c >> @@ -48,6 +48,7 @@ static int load_elf_binary(struct linux_ >> static int load_elf_library(struct file *); >> static unsigned long elf_map(struct file *, unsigned long, struct elf_phdr *, >> int, int, unsigned long); >> +static void randomize_stack_user(unsigned char *buf, size_t nbytes); >> >> /* >> * If we don't support core dumping, then supply a NULL so we >> @@ -200,7 +201,7 @@ create_elf_tables(struct linux_binprm *b >> /* >> * Generate 16 random bytes for userspace PRNG seeding. >> */ >> - get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes)); >> + randomize_stack_user(k_rand_bytes, sizeof(k_rand_bytes)); >> u_rand_bytes = (elf_addr_t __user *) >> STACK_ALLOC(p, sizeof(k_rand_bytes)); >> if (__copy_to_user(u_rand_bytes, k_rand_bytes, sizeof(k_rand_bytes))) >> @@ -558,6 +559,29 @@ static unsigned long randomize_stack_top >> #endif >> } >> >> +/* >> + * A wrapper of get_random_int() to generate random bytes which has lower >> + * overhead than call get_random_bytes() directly. >> + * create_elf_tables() call this function to generate 16 random bytes for >> + * userspace PRNG seeding. >> + */ >> +static void randomize_stack_user(unsigned char *buf, size_t nbytes) >> +{ >> + unsigned char *p = buf; >> + >> + while (nbytes) { >> + unsigned int random_variable; >> + size_t chunk = min(nbytes, sizeof(unsigned int)); >> + >> + random_variable = get_random_int() & STACK_RND_MASK; >> + random_variable <<= PAGE_SHIFT; > > Why is this using STACK_RND_MASK? That's not sensible. And the shift > is especially odd. AIUI, these two lines should just be: Definitely! I copied it from randomize_stack_top(), it's not proper to use here, will fix it accordingly.
Thanks, -Jeff > > random_variable = get_random_int(); > >> + >> + memcpy(p, &random_variable, chunk); >> + p += chunk; >> + nbytes -= chunk; >> + } >> +} >> + >> static int load_elf_binary(struct linux_binprm *bprm) >> { >> struct file *interpreter = NULL; /* to shut gcc up */ >> _ >> >> Patches currently in -mm which might be from jeff.liu@oracle.com are >> >> documentation-cgroups-memorytxt-s-mem_cgroup_charge-mem_cgroup_change_common.patch >> mm-vmscanc-try_to_freeze-returns-boolean.patch >> binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patch >> binfmt_elfc-use-get_random_int-to-fix-entropy-depleting-fix.patch >> > > -Kees >
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