Re: [PATCH V3] binfmt_elf.c: Introduce a wrapper of get_random_int()to fix entropy depleting

[Jeff Liu]

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@xxxxxxxxxx>
> Cc: John Sobecki <john.sobecki@xxxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> 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>
> 
> ---
>  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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