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Groups > linux.kernel > #1666305

[PATCH] random: silence compiler warnings and fix race

From "Jason A. Donenfeld" <Jason@zx2c4.com>
Newsgroups linux.kernel
Subject [PATCH] random: silence compiler warnings and fix race
Date 2017-06-15 00:50 +0200
Message-ID <tSpgR-2GV-13@gated-at.bofh.it> (permalink)
References <tSmj0-Qa-5@gated-at.bofh.it>
Organization linux.* mail to news gateway

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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.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Ted -- the first part of this is the fixup patch we discussed earlier.
Then I added on top a fix for a potentially related race.

I'm not totally convinced that moving this block to outside the spinlock
is 100% okay, so please give this a close look before merging.


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

diff --git a/drivers/char/random.c b/drivers/char/random.c
index e870f329db88..01a260f67437 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -803,13 +803,13 @@ static int crng_fast_load(const char *cp, size_t len)
 		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_state *crng, struct entropy_store *r)
 	}
 	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_state *crng, struct entropy_store *r)
 		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)
@@ -2041,8 +2041,8 @@ static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u64);
 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
@@ -2073,8 +2073,8 @@ static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u32);
 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))
-- 
2.13.1

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Thread

[PATCH v5 00/13] Unseeded In-Kernel Randomness Fixes "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 13/13] random: warn when kernel uses unseeded randomness "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 06/13] iscsi: ensure RNG is seeded before use "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 05/13] crypto/rng: ensure that the RNG is ready before using "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 11/13] net/route: use get_random_int for random counter "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 10/13] net/neighbor: use get_random_u32 for 32-bit hash random "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 08/13] cifs: use get_random_u32 for 32-bit lock random "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 12/13] bluetooth/smp: ensure RNG is properly seeded before ECDH use "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 02/13] random: add synchronous API for the urandom pool "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 01/13] random: invalidate batched entropy after crng init "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
    Re: [PATCH v5 01/13] random: invalidate batched entropy after crng  init Sebastian Andrzej Siewior <sebastian@breakpoint.cc> - 2017-06-14 21:40 +0200
      Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-15 00:40 +0200
        Re: [PATCH v5 01/13] random: invalidate batched entropy after crng  init Sebastian Andrzej Siewior <sebastian@breakpoint.cc> - 2017-06-16 10:40 +0200
          Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-16 14:20 +0200
            Re: [PATCH v5 01/13] random: invalidate batched entropy after crng  init Sebastian Andrzej Siewior <sebastian@breakpoint.cc> - 2017-06-16 16:40 +0200
      [PATCH] random: silence compiler warnings and fix race "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-15 00:50 +0200
        Re: [PATCH] random: silence compiler warnings and fix race Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-06-16 16:40 +0200
          Re: [PATCH] random: silence compiler warnings and fix race "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-17 02:40 +0200
            Re: [PATCH] random: silence compiler warnings and fix race Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-06-19 09:50 +0200
              Re: [PATCH] random: silence compiler warnings and fix race "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-19 23:00 +0200
        Re: [PATCH] random: silence compiler warnings and fix race "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-19 23:00 +0200
  [PATCH v5 03/13] random: add get_random_{bytes,u32,u64,int,long,once}_wait family "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 04/13] security/keys: ensure RNG is seeded before use "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200

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