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random: fix data race on crng_node_pool 

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: 1e7f583af6 ("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>
This commit is contained in:
Eric Biggers 2021-12-20 16:41:56 -06:00 committed by Jason A. Donenfeld
parent 5320eb42de
commit 5d73d1e320
1 changed files with 22 additions and 20 deletions
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42
drivers/char/random.c
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@ -854,8 +854,8 @@ 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);
@ -868,8 +868,26 @@ 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
/*
@ -1016,15 +1034,7 @@ 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);
}
/*
@ -1053,15 +1063,7 @@ 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)