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Message-ID: <CAGXu5jLpE5kTGGqLqDRwRaEOt+nhpKc52jQFs+rsa+0GVKA-vw@mail.gmail.com>
Date: Tue, 6 Nov 2012 22:21:23 -0800
From: Kees Cook <keescook@...omium.org>
To: Jeff Liu <jeff.liu@...cle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Andreas Dilger <aedilger@...il.com>,
John Sobecki <john.sobecki@...cle.com>,
"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
Alan Cox <alan@...ux.intel.com>,
"arnd@...db.de" <arnd@...db.de>,
James Morris <james.l.morris@...cle.com>,
"Ted Ts'o" <tytso@....edu>,
"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
jakub@...hat.com, drepper@...hat.com,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [RESEND PATCH V3] binfmt_elf.c: use get_random_int() to fix
entropy depleting
On Tue, Nov 6, 2012 at 10:11 PM, Jeff Liu <jeff.liu@...cle.com> wrote:
> Hello,
>
> This is the revised patch for fix entropy depleting.
>
> Changes:
> --------
> v3->v2:
> - Tweak code comments of random_stack_user().
> - Remove redundant bits mask and shift upon the random variable.
>
> v2->v1:
> Fix random copy to check up buffer length that are not 4-byte multiples.
>
> v2 can be found at:
> http://www.spinics.net/lists/linux-fsdevel/msg59418.html
> v1 can be found at:
> http://www.spinics.net/lists/linux-fsdevel/msg59128.html
>
> Many thanks to Andreas, Andrew as well as Kees for reviewing the patch of past!
> -Jeff
>
>
> 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 | 22 +++++++++++++++++++++-
> 1 files changed, 21 insertions(+), 1 deletions(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index fbd9f60..b6c59f6 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -48,6 +48,7 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs);
> 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);
I think it would be easier to just move the function ahead of its use
to avoid the predeclaration.
>
> /*
> * If we don't support core dumping, then supply a NULL so we
> @@ -200,7 +201,7 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
> /*
> * 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 +559,25 @@ static unsigned long randomize_stack_top(unsigned long stack_top)
> #endif
> }
>
> +/*
> + * Use get_random_int() to implement AT_RANDOM while avoiding depletion
> + * of the entropy pool.
> + */
> +static void randomize_stack_user(unsigned char *buf, size_t nbytes)
I think this name needs changing -- it has nothing to do with the
stack except that that's where it ends up in userspace. Perhaps
"get_atrandom_bytes"?
> +{
> + unsigned char *p = buf;
> +
> + while (nbytes) {
> + unsigned int random_variable;
> + size_t chunk = min(nbytes, sizeof(unsigned int));
> +
> + random_variable = get_random_int();
I still want to hear at least from Ted about this changes -- we would
be potentially increasing the predictability of these bytes...
> + memcpy(p, &random_variable, chunk);
> + p += chunk;
> + nbytes -= chunk;
> + }
> +}
> +
> static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
> {
> struct file *interpreter = NULL; /* to shut gcc up */
> --
> 1.7.4.1
-Kees
--
Kees Cook
Chrome OS Security
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