From: Eric Biggers ebiggers@google.com

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 --- 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 605969ed0f96..1294b4527cdd 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 READ_ONCE() 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 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

/* @@ -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)