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Message-ID: <CAGXu5jJ779UvpZwO-fi7-m+i=OfqwgXh1PC4GNwjhfD9GD82wQ@mail.gmail.com>
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
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