[RFC PATCH 28/41] random: don't award entropy to disk + input events if in FIPS mode

[Nicolai Stange]

NIST SP800-90C prohibits the use of multiple correlated entropy sources.

Obviously, add_disk_randomness(), add_input_randomness() and
add_interrupt_randomness() are not independent.

Follow the approach taken by Stephan Müller's LRNG patchset ([1]) and don't
award any entropy to the former two if fips_enabled is true.
Note that the entropy loss has already been compensated for by a previous
patch increasing the IRQ event estimate.

The actual entropy accounting from add_disk_randomness() and
add_input_randomness() is implemented in the common add_timer_randomness()
called therefrom.

Make the latter to not dispatch any entropy to the global entropy balance
if fips_enabled is on.

[1] https://lkml.kernel.org/r/5695397.lOV4Wx5bFT@xxxxxxxxxxxxxxxxxxx

Suggested-by: Stephan Müller <smueller@xxxxxxxxxx>
Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
---
 drivers/char/random.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 8f79e90f2429..680ccc82a436 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1481,12 +1481,24 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 
 	r = &input_pool;
 	spin_lock_irqsave(&r->lock, flags);
-	/*
-	 * delta is now minimum absolute delta.
-	 * Round down by 1 bit on general principles,
-	 * and limit entropy estimate to 12 bits.
-	 */
-	__queue_entropy(r, &q, min_t(int, fls(delta>>1), 11) << ENTROPY_SHIFT);
+	if (!fips_enabled) {
+		unsigned int nfrac;
+
+		/*
+		 * delta is now minimum absolute delta.
+		 * Round down by 1 bit on general principles,
+		 * and limit entropy estimate to 12 bits.
+		 */
+		nfrac = min_t(int, fls(delta>>1), 11) << ENTROPY_SHIFT;
+		__queue_entropy(r, &q, nfrac);
+	} else {
+		/*
+		 * Multiple correlated entropy sources are prohibited
+		 * by NIST SP800-90C. Leave it up to
+		 * add_interrupt_randomness() to contribute any
+		 * entropy.
+		 */
+	}
 	__mix_pool_bytes(r, &sample, sizeof(sample));
 	reseed = __dispatch_queued_entropy_fast(r, &q);
 	spin_unlock_irqrestore(&r->lock, flags);
-- 
2.26.2