Re: + binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patchadded to -mm tree

[Kees Cook]

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@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> 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@xxxxxxxxxx>
> Cc: John Sobecki <john.sobecki@xxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Andreas Dilger <aedilger@xxxxxxxxx>
> Cc: Alan Cox <alan@xxxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnn@xxxxxxxx>
> Cc: James Morris <james.l.morris@xxxxxxxxxx>
> Cc: Ted Ts'o <tytso@xxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Jakub Jelinek <jakub@xxxxxxxxxx>
> Cc: Ulrich Drepper <drepper@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
>  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@xxxxxxxxxx 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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