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From: Eric Biggers <ebiggers@kernel.org>
To: Theodore Ts'o <tytso@mit.edu>
Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
	stable@vger.kernel.org
Subject: [PATCH] random: use correct memory barriers for crng_node_pool
Date: Wed, 16 Sep 2020 16:30:42 -0700
Message-ID: <20200916233042.51634-1-ebiggers@kernel.org> (raw)

From: Eric Biggers <ebiggers@google.com>

When a CPU selects which CRNG to use, it accesses crng_node_pool without
a memory barrier.  That's wrong, because crng_node_pool can be set by
another CPU concurrently.  Without a memory barrier, the crng_state that
is used might not appear to be fully initialized.

There's an explicit mb() on the write side, but it's redundant with
cmpxchg() (or cmpxchg_release()) and does nothing to fix the read side.

Implement this correctly by using a cmpxchg_release() +
smp_load_acquire() pair.

Fixes: 1e7f583af67b ("random: make /dev/urandom scalable for silly userspace programs")
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 drivers/char/random.c | 42 ++++++++++++++++++++++--------------------
 1 file changed, 22 insertions(+), 20 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 09b1551d4092f..9f1e7a4a0fbbb 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -843,8 +843,8 @@ static void do_numa_crng_init(struct work_struct *work)
 		crng_initialize_secondary(crng);
 		pool[i] = crng;
 	}
-	mb();
-	if (cmpxchg(&crng_node_pool, NULL, pool)) {
+	/* pairs with smp_load_acquire() in select_crng() */
+	if (cmpxchg_release(&crng_node_pool, NULL, pool) != NULL) {
 		for_each_node(i)
 			kfree(pool[i]);
 		kfree(pool);
@@ -857,8 +857,26 @@ static void numa_crng_init(void)
 {
 	schedule_work(&numa_crng_init_work);
 }
+
+static inline struct crng_state *select_crng(void)
+{
+	struct crng_state **pool;
+	int nid = numa_node_id();
+
+	/* pairs with cmpxchg_release() in do_numa_crng_init() */
+	pool = smp_load_acquire(&crng_node_pool);
+	if (pool && pool[nid])
+		return pool[nid];
+
+	return &primary_crng;
+}
 #else
 static void numa_crng_init(void) {}
+
+static inline struct crng_state *select_crng(void)
+{
+	return &primary_crng;
+}
 #endif
 
 /*
@@ -1005,15 +1023,7 @@ static void _extract_crng(struct crng_state *crng,
 
 static void extract_crng(__u8 out[CHACHA_BLOCK_SIZE])
 {
-	struct crng_state *crng = NULL;
-
-#ifdef CONFIG_NUMA
-	if (crng_node_pool)
-		crng = crng_node_pool[numa_node_id()];
-	if (crng == NULL)
-#endif
-		crng = &primary_crng;
-	_extract_crng(crng, out);
+	_extract_crng(select_crng(), out);
 }
 
 /*
@@ -1042,15 +1052,7 @@ static void _crng_backtrack_protect(struct crng_state *crng,
 
 static void crng_backtrack_protect(__u8 tmp[CHACHA_BLOCK_SIZE], int used)
 {
-	struct crng_state *crng = NULL;
-
-#ifdef CONFIG_NUMA
-	if (crng_node_pool)
-		crng = crng_node_pool[numa_node_id()];
-	if (crng == NULL)
-#endif
-		crng = &primary_crng;
-	_crng_backtrack_protect(crng, tmp, used);
+	_crng_backtrack_protect(select_crng(), tmp, used);
 }
 
 static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
-- 
2.28.0


             reply index

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-16 23:30 Eric Biggers [this message]
2020-09-17  7:26 ` Herbert Xu
2020-09-17 16:58   ` Eric Biggers
2020-09-21  8:19     ` Herbert Xu
2020-09-21 15:27       ` Paul E. McKenney
2020-09-21 22:11         ` Herbert Xu
2020-09-21 23:26           ` Paul E. McKenney
2020-09-21 23:51             ` Herbert Xu
2020-09-22 18:42               ` Paul E. McKenney
2020-09-22 18:59                 ` Eric Biggers
2020-09-22 20:31                   ` Paul E. McKenney
2020-09-21 23:52             ` Eric Biggers
2020-09-22 18:31               ` Paul E. McKenney
2020-09-22 19:09                 ` Eric Biggers
2020-09-22 20:56                   ` Paul E. McKenney
2020-09-22 21:55                     ` Eric Biggers
2020-09-25  0:59                       ` Paul E. McKenney
2020-09-25  2:09                         ` Eric Biggers
2020-09-25  3:31                           ` Paul E. McKenney
2020-10-02  3:07                             ` Eric Biggers
2020-10-08 18:31                               ` Paul E. McKenney

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