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

Greg Kroah-Hartman
2022-06-23T16:47:16Z

From: Eric Biggers <ebiggers@google.com>

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@kernel.org/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@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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

/*