Subject: [PATCH] random: wait for system_wq before pushing entropy into output
 pools
From: Konstantin Khlebnikov <k.khlebnikov@samsung.com>
To: Tejun Heo <tj@kernel.org>, "Theodore Ts'o" <tytso@mit.edu>,
        linux-kernel@vger.kernel.org
Cc: Andrey Ryabinin <a.ryabinin@samsung.com>,
        Dmitry Vyukov <dvyukov@google.com>
Date: Thu, 20 Nov 2014 16:54:21 +0400
Message-id: <20141120135421.3819.24114.stgit@buzz>
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Sender: linux-kernel-owner@vger.kernel.org

First prototype of tool which injects random delays in random places
(implemented on top of kernel address sanitizer by Andrey Ryabinin)
have caught race right at the first try.

credit_entropy_bits() schedules work for pushing entropy from one pool
into another too early, when system_wq isn't yet created. It's called from
add_interrupt_randomness, so this path starts working right after enabling
interrupts in start_kernel, but system workqueues are initialized much
later in early_initcall(init_workqueues). It's impossible to create them
earlier because threads have to be forked from kthreadd.

This patch skips pushing if system_wq isn't here yet. Delaying this till
SYSTEM_RUNNING state might lead to significant lost of precious entropy.

Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@samsung.com>
Reported-and-tested-by: Andrey Ryabinin <a.ryabinin@samsung.com>
---
 drivers/char/random.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 04645c0..d4a684f 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -683,7 +683,7 @@ retry:
 		 * full.
 		 */
 		if (entropy_bits > random_write_wakeup_bits &&
-		    r->initialized &&
+		    system_wq && r->initialized &&
 		    r->entropy_total >= 2*random_read_wakeup_bits) {
 			static struct entropy_store *last = &blocking_pool;
 			struct entropy_store *other = &blocking_pool;
@@ -695,7 +695,7 @@ retry:
 				last = other;
 			if (last->entropy_count <=
 			    3 * last->poolinfo->poolfracbits / 4) {
-				schedule_work(&last->push_work);
+				queue_work(system_wq, &last->push_work);
 				r->entropy_total = 0;
 			}
 		}

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