linux-kernel - [PATCH] random: Don't freeze in add_hwgenerator

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Date:   Sun, 10 Nov 2019 14:55:42 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Keerthy <j-keerthy@...com>, Stephen Boyd <swboyd@...omium.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] random: Don't freeze in add_hwgenerator_randomness() if stopping kthread

Since commit 59b569480dc8
("random: Use wait_event_freezable() in add_hwgenerator_randomness()")
there is a race in add_hwgenerator_randomness() between freezing and
stopping the calling kthread.

This commit changed wait_event_interruptible() call with
kthread_freezable_should_stop() as a condition into wait_event_freezable()
with just kthread_should_stop() as a condition to fix a warning that
kthread_freezable_should_stop() might sleep inside the wait.

wait_event_freezable() ultimately calls __refrigerator() with its
check_kthr_stop argument set to false, which causes it to keep the kthread
frozen even if somebody calls kthread_stop() on it.

Calling wait_event_freezable() with kthread_should_stop() as a condition
is racy because it doesn't take into account the situation where this
condition becomes true on a kthread marked for freezing only after this
condition has already been checked.

Calling freezing() should avoid the issue that the commit 59b569480dc8 has
fixed, as it is only a checking function, it doesn't actually do the
freezing.

add_hwgenerator_randomness() has two post-boot users: in khwrng the
kthread will be frozen anyway by call to kthread_freezable_should_stop()
in its main loop, while its second user (ath9k-hwrng) is not freezable at
all.

This change allows a VM with virtio-rng loaded to write s2disk image
successfully.

Fixes: 59b569480dc8 ("random: Use wait_event_freezable() in add_hwgenerator_randomness()")
Signed-off-by: Maciej S. Szmigiero <mail@...iej.szmigiero.name>
---
 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 de434feb873a..2f87910dd498 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2500,8 +2500,8 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 	 * We'll be woken up again once below random_write_wakeup_thresh,
 	 * or when the calling thread is about to terminate.
 	 */
-	wait_event_freezable(random_write_wait,
-			kthread_should_stop() ||
+	wait_event_interruptible(random_write_wait,
+			kthread_should_stop() || freezing(current) ||
 			ENTROPY_BITS(&input_pool) <= random_write_wakeup_bits);
 	mix_pool_bytes(poolp, buffer, count);
 	credit_entropy_bits(poolp, entropy);