Re: [PATCH V2] binfmt_elf.c: Introduce a wrapper of get_random_int() to fix entropy depleting

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 01 Nov 2012 15:22:51 +0800
Jeff Liu <jeff.liu@oracle.com> wrote:

> Hello,
> 
> Entropy quickly depleting under normal I/O operations like ls(1), cat(1), etc...
> between 2.6.30 to current mainline,

Well that's bad.  Let's cc Kees, who broke it ;)

> 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 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.
> 
> 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
>
> ...
>
> --- 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 +559,29 @@ 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 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;
> +
> +		memcpy(p, &random_variable, chunk);
> +		p += chunk;
> +		nbytes -= chunk;
> +	}
> +}

Prior to f06295b44c296 ("ELF: implement AT_RANDOM for glibc PRNG
seeding"), glibc was opening and using /dev/urandom for this.  So
presumably the urandom level of security was sufficient.

Or perhaps it wasn't and the stronger get_random_bytes() works better -
I don't know?

>From my reading of the source, get_random_int() is weaker even than
/dev/urandom?


So my bottom line is: I don't know!  Kees?  Ted?  Ulrich?