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Date: Tue, 6 Nov 2012 17:11:17 -0800 From: Kees Cook <keescook@...omium.org> To: akpm@...ux-foundation.org Cc: jeff.liu@...cle.com, aedilger@...il.com, alan@...ux.intel.com, arnn@...db.de, drepper@...hat.com, gregkh@...uxfoundation.org, jakub@...hat.com, james.l.morris@...cle.com, john.sobecki@...cle.com, tytso@....edu, viro@...iv.linux.org.uk, LKML <linux-kernel@...r.kernel.org> Subject: Re: + binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patch added to -mm tree 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. Notes below... On Tue, Nov 6, 2012 at 4:16 PM, <akpm@...ux-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@...cle.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@...cle.com> > Cc: John Sobecki <john.sobecki@...cle.com> > Cc: Al Viro <viro@...iv.linux.org.uk> > Cc: Andreas Dilger <aedilger@...il.com> > Cc: Alan Cox <alan@...ux.intel.com> > Cc: Arnd Bergmann <arnn@...db.de> > Cc: James Morris <james.l.morris@...cle.com> > Cc: Ted Ts'o <tytso@....edu> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org> > Cc: Kees Cook <keescook@...omium.org> > Cc: Jakub Jelinek <jakub@...hat.com> > Cc: Ulrich Drepper <drepper@...hat.com> > Signed-off-by: Andrew Morton <akpm@...ux-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: 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@...cle.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 -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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