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Date: Wed, 07 Nov 2012 13:52:56 +0800
From: Jeff Liu <jeff.liu@...cle.com>
To: LKML <linux-kernel@...r.kernel.org>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook <keescook@...omium.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: [PATCH V3] binfmt_elf.c: Introduce a wrapper of get_random_int()
to fix entropy depleting
Please ignore this patch since I forgot revising the comments of this
new wrapper according to Andrew's, will re-send it a little while, sorry
for the noise!
-Jeff
On 11/07/2012 01:27 PM, Jeff Liu wrote:
> Hello,
>
> We have observed entropy quickly depleting under normal I/O between 2.6.30 to upstream, 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
>
> It has been occurred with fs/binfmt_elf.c: create_elf_tables()->get_random_bytes()
> was introduced began at 2.6.30.
> /*
> * Generate 16 random bytes for userspace PRNG seeding.
> */
> get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes));
>
> This proposal patch is trying to introduce a wrapper of get_random_int() which has lower overhead
> than calling get_random_bytes() directly.
>
> Entropy increased 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
>
>
> v3->v2:
> -------
> Remove redundant bits mask and shift upon the random variable according to Kees's review.
>
> v2->v1:
> -------
> Fix random copy to check up buffer length that are not 4-byte multiples according to Andreas's comments.
>
> 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
>
>
> Signed-off-by: Jie Liu <jeff.liu@...cle.com>
> Cc: John Sobecki <john.sobecki@...cle.com>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> 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>
>
> ---
> fs/binfmt_elf.c | 24 +++++++++++++++++++++++-
> 1 files changed, 23 insertions(+), 1 deletions(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index fbd9f60..9c36e50 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);
>
> /*
> * 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 +559,27 @@ static unsigned long randomize_stack_top(unsigned long stack_top)
> #endif
> }
>
> +/*
> + * A wrapper of get_random_int() to generate random bytes which has lower
> + * overhead than calling 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();
> + 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 */
>
--
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