[PATCH RESEND] random: use correct memory barriers for crng_node_pool

Eric Biggers
2021-12-19T02:52:11Z

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.

Note: READ_ONCE() could be used instead of smp_load_acquire(), but it is
harder to verify that it is correct, so I'd prefer not to use it here.

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>
---

I sent this fix about a year ago
(https://lore.kernel.org/lkml/20200916233042.51634-1-ebiggers@kernel.org/T/#u),
and though it's a correct fix, it was derailed by a debate about whether
it's safe to use READ_ONCE() instead of smp_load_acquire() or not.
Therefore, the current code, which (AFAIK) everyone agrees is buggy, was
never actually fixed. Since random.c has a new maintainer now, I think
it's worth sending this fix for reconsideration.

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..349a6f235c61 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)
--
2.34.1