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SubjectRe: [PATCH v4 3/3] vsprintf: Use hw RNG for ptr_key
On Tue, 15 May 2018 13:06:26 +1000
"Tobin C. Harding" <me@tobin.cc> 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@chromium.org>
> Signed-off-by: Tobin C. Harding <me@tobin.cc>
> ---
> 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@goodmis.org>

-- Steve

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