[PATCH 4.9 025/264] random: fix data race on crng_node_pool

[Greg Kroah-Hartman]

From: Eric Biggers <ebiggers@xxxxxxxxxx>

commit 5d73d1e320c3fd94ea15ba5f79301da9a8bcc7de upstream.

extract_crng() and crng_backtrack_protect() load crng_node_pool with a
plain load, which causes undefined behavior if do_numa_crng_init()
modifies it concurrently.

Fix this by using READ_ONCE().  Note: as per the previous discussion
https://lore.kernel.org/lkml/20211219025139.31085-1-ebiggers@xxxxxxxxxx/T/#u,
READ_ONCE() is believed to be sufficient here, and it was requested that
it be used here instead of smp_load_acquire().

Also change do_numa_crng_init() to set crng_node_pool using
cmpxchg_release() instead of mb() + cmpxchg(), as the former is
sufficient here but is more lightweight.

Fixes: 1e7f583af67b ("random: make /dev/urandom scalable for silly userspace programs")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/char/random.c |   22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -814,8 +814,8 @@ static void do_numa_crng_init(struct wor
 		crng_initialize(crng);
 		pool[i] = crng;
 	}
-	mb();
-	if (cmpxchg(&crng_node_pool, NULL, pool)) {
+	/* pairs with READ_ONCE() in select_crng() */
+	if (cmpxchg_release(&crng_node_pool, NULL, pool) != NULL) {
 		for_each_node(i)
 			kfree(pool[i]);
 		kfree(pool);
@@ -828,8 +828,26 @@ static void numa_crng_init(void)
 {
 	schedule_work(&numa_crng_init_work);
 }
+
+static 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 = READ_ONCE(crng_node_pool);
+	if (pool && pool[nid])
+		return pool[nid];
+
+	return &primary_crng;
+}
 #else
 static void numa_crng_init(void) {}
+
+static struct crng_state *select_crng(void)
+{
+	return &primary_crng;
+}
 #endif
 
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