Re: [PATCH v4 3/3] vsprintf: Use hw RNG for ptr_key

[Steven Rostedt]

On Tue, 15 May 2018 13:06:26 +1000
"Tobin C. Harding" <me@xxxxxxxx> wrote:

> Currently we must wait for enough entropy to become available before
> hashed pointers can be printed.  We can remove this wait by using the
> hw RNG if available.
> 
> Use hw RNG to get keying material.
> 
> Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Tobin C. Harding <me@xxxxxxxx>
> ---
>  lib/vsprintf.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index b82f0c6c2aec..3697a19c2b25 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1657,9 +1657,8 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
>  static bool have_filled_random_ptr_key __read_mostly;
>  static siphash_key_t ptr_key __read_mostly;
>  
> -static void fill_random_ptr_key(struct random_ready_callback *unused)
> +static void ptr_key_ready(void)
>  {
> -	get_random_bytes(&ptr_key, sizeof(ptr_key));
>  	/*
>  	 * have_filled_random_ptr_key==true is dependent on get_random_bytes().
>  	 * ptr_to_id() needs to see have_filled_random_ptr_key==true

Nothing to do with this patch, but I believe there's a missing memory
barrier in the code.

Right after this we have:

	smp_mb();
	WRITE_ONCE(have_filled_random_ptr_key, true);

Where the comment says that have_filled_random_ptr_key must be set
after ptr_key has been updated. But there's no memory barrier on the
read side. In fact, I think this could be a smp_wmb() instead of a
smp_mb(). The read side has:

	if (unlikely(!have_filled_random_ptr_key))
		return string(buf, end, "(ptrval)", spec);

/* Missing memory barrier smp_rmb() here. */

	hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);

Thus we can have something like:

	CPU0				CPU1
	----				----
				   load ptr_key = 0
   store ptr_key = random
   smp_mb()
   store have_filled_random_ptr_key

				   load have_filled_random_ptr_key = true

				    BAD BAD BAD!

I'll send a patch.

> @@ -1669,14 +1668,28 @@ static void fill_random_ptr_key(struct random_ready_callback *unused)
>  	WRITE_ONCE(have_filled_random_ptr_key, true);
>  }
>  
> +static void fill_random_ptr_key(struct random_ready_callback *unused)
> +{
> +	get_random_bytes(&ptr_key, sizeof(ptr_key));
> +	ptr_key_ready();
> +}
> +
>  static struct random_ready_callback random_ready = {
>  	.func = fill_random_ptr_key
>  };
>  
>  static int __init initialize_ptr_random(void)
>  {
> -	int ret = add_random_ready_callback(&random_ready);
> +	int ret;
> +	int key_size = sizeof(ptr_key);
> +
> +	/* Use hw RNG if available */
> +	if (get_random_bytes_arch(&ptr_key, key_size) == key_size) {
> +		ptr_key_ready();
> +		return 0;
> +	}
>  
> +	ret = add_random_ready_callback(&random_ready);
>  	if (!ret) {
>  		return 0;
>  	} else if (ret == -EALREADY) {

But this patch looks good.

Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

-- Steve