Re: [PATCH] [char] random: fix priming of last_data

[Neil Horman <nhorman@tuxdriver.com>]

On Tue, Mar 19, 2013 at 12:18:09PM -0400, Jarod Wilson wrote:
> Commit ec8f02da9e added priming of last_data per fips requirements.
> Unfortuantely, it did so in a way that can lead to multiple threads all
> incrementing nbytes, but only one actually doing anything with the extra
> data, which leads to some fun  random corruption and panics.
> 
> The fix is to simply do everything needed to prime last_data in a single
> shot, so there's no window for multiple cpus to increment nbytes -- in
> fact, we won't even increment or decrement nbytes anymore, we'll just
> extract the needed EXTRACT_BYTES one time per pool and then carry on with
> the normal routine.
> 
> All these changes have been tested across multiple hosts and architectures
> where panics were previously encoutered. The code changes are are strictly
> limited to areas only touched when when booted in fips mode.
> 
> This change should also go into 3.8-stable, to make the myriads of fips
> users on 3.8.x happy.
> 
> Tested-by: Jan Stancek <jstancek@redhat.com>
> Tested-by: Jan Stodola <jstodola@redhat.com>
> CC: Herbert Xu <herbert@gondor.apana.org.au>
> CC: Neil Horman <nhorman@tuxdriver.com>
> CC: "David S. Miller" <davem@davemloft.net>
> CC: Matt Mackall <mpm@selenic.com>
> CC: "Theodore Ts'o" <tytso@mit.edu>
> CC: linux-crypto@vger.kernel.org
> CC: stable@vger.kernel.org
> Signed-off-by: Jarod Wilson <jarod@redhat.com>
> ---
>  drivers/char/random.c |   30 +++++++++++++++---------------
>  1 files changed, 15 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 85e81ec..15e5a2b 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -953,10 +953,23 @@ static ssize_t extract_entropy(struct entropy_store *r, void *buf,
>  {
>  	ssize_t ret = 0, i;
>  	__u8 tmp[EXTRACT_SIZE];
> +	unsigned long flags;
>  
>  	/* if last_data isn't primed, we need EXTRACT_SIZE extra bytes */
> -	if (fips_enabled && !r->last_data_init)
> -		nbytes += EXTRACT_SIZE;
> +	if (fips_enabled) {
> +		spin_lock_irqsave(&r->lock, flags);
> +		if (!r->last_data_init) {
> +			r->last_data_init = true;
> +			spin_unlock_irqrestore(&r->lock, flags);
> +			trace_extract_entropy(r->name, EXTRACT_SIZE,
> +					      r->entropy_count, _RET_IP_);
> +			xfer_secondary_pool(r, EXTRACT_SIZE);
> +			extract_buf(r, tmp);
> +			spin_lock_irqsave(&r->lock, flags);
> +			memcpy(r->last_data, tmp, EXTRACT_SIZE);
> +		}
> +		spin_unlock_irqrestore(&r->lock, flags);
> +	}
>  
>  	trace_extract_entropy(r->name, nbytes, r->entropy_count, _RET_IP_);
>  	xfer_secondary_pool(r, nbytes);
> @@ -966,19 +979,6 @@ static ssize_t extract_entropy(struct entropy_store *r, void *buf,
>  		extract_buf(r, tmp);
>  
>  		if (fips_enabled) {
> -			unsigned long flags;
> -
> -
> -			/* prime last_data value if need be, per fips 140-2 */
> -			if (!r->last_data_init) {
> -				spin_lock_irqsave(&r->lock, flags);
> -				memcpy(r->last_data, tmp, EXTRACT_SIZE);
> -				r->last_data_init = true;
> -				nbytes -= EXTRACT_SIZE;
> -				spin_unlock_irqrestore(&r->lock, flags);
> -				extract_buf(r, tmp);
> -			}
> -
>  			spin_lock_irqsave(&r->lock, flags);
>  			if (!memcmp(tmp, r->last_data, EXTRACT_SIZE))
>  				panic("Hardware RNG duplicated output!\n");
> -- 
> 1.7.1
> 
> 
Acked-by: Neil Horman <nhorman@tuxdriver.com>