linux-kernel - [PATCH 4.11 30/58] random: silence compiler warnings and fix race

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Date:   Tue, 27 Jun 2017 16:12:33 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Theodore Tso <tytso@....edu>
Subject: [PATCH 4.11 30/58] random: silence compiler warnings and fix race

4.11-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jason A. Donenfeld <Jason@...c4.com>

commit 4a072c71f49b0a0e495ea13423bdb850da73c58c upstream.

Odd versions of gcc for the sh4 architecture will actually warn about
flags being used while uninitialized, so we set them to zero. Non crazy
gccs will optimize that out again, so it doesn't make a difference.

Next, over aggressive gccs could inline the expression that defines
use_lock, which could then introduce a race resulting in a lock
imbalance. By using READ_ONCE, we prevent that fate. Finally, we make
that assignment const, so that gcc can still optimize a nice amount.

Finally, we fix a potential deadlock between primary_crng.lock and
batched_entropy_reset_lock, where they could be called in opposite
order. Moving the call to invalidate_batched_entropy to outside the lock
rectifies this issue.

Fixes: b169c13de473a85b3c859bb36216a4cb5f00a54a
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
Signed-off-by: Theodore Ts'o <tytso@....edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/char/random.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -803,13 +803,13 @@ static int crng_fast_load(const char *cp
 		p[crng_init_cnt % CHACHA20_KEY_SIZE] ^= *cp;
 		cp++; crng_init_cnt++; len--;
 	}
+	spin_unlock_irqrestore(&primary_crng.lock, flags);
 	if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
 		invalidate_batched_entropy();
 		crng_init = 1;
 		wake_up_interruptible(&crng_init_wait);
 		pr_notice("random: fast init done\n");
 	}
-	spin_unlock_irqrestore(&primary_crng.lock, flags);
 	return 1;
 }

@@ -841,6 +841,7 @@ static void crng_reseed(struct crng_stat
 	}
 	memzero_explicit(&buf, sizeof(buf));
 	crng->init_time = jiffies;
+	spin_unlock_irqrestore(&primary_crng.lock, flags);
 	if (crng == &primary_crng && crng_init < 2) {
 		invalidate_batched_entropy();
 		crng_init = 2;
@@ -848,7 +849,6 @@ static void crng_reseed(struct crng_stat
 		wake_up_interruptible(&crng_init_wait);
 		pr_notice("random: crng init done\n");
 	}
-	spin_unlock_irqrestore(&primary_crng.lock, flags);
 }

 static inline void crng_wait_ready(void)
@@ -2037,8 +2037,8 @@ static DEFINE_PER_CPU(struct batched_ent
 u64 get_random_u64(void)
 {
 	u64 ret;
-	bool use_lock = crng_init < 2;
-	unsigned long flags;
+	bool use_lock = READ_ONCE(crng_init) < 2;
+	unsigned long flags = 0;
 	struct batched_entropy *batch;

 #if BITS_PER_LONG == 64
@@ -2069,8 +2069,8 @@ static DEFINE_PER_CPU(struct batched_ent
 u32 get_random_u32(void)
 {
 	u32 ret;
-	bool use_lock = crng_init < 2;
-	unsigned long flags;
+	bool use_lock = READ_ONCE(crng_init) < 2;
+	unsigned long flags = 0;
 	struct batched_entropy *batch;

 	if (arch_get_random_int(&ret))