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 /*
