From: Jakub Sitnicki jakub@cloudflare.com
[ Upstream commit 75e68e5bf2c7fa9d3e874099139df03d5952a3e1 ]
We can end up modifying the sockhash bucket list from two CPUs when a sockhash is being destroyed (sock_hash_free) on one CPU, while a socket that is in the sockhash is unlinking itself from it on another CPU it (sock_hash_delete_from_link).
This results in accessing a list element that is in an undefined state as reported by KASAN:
| ================================================================== | BUG: KASAN: wild-memory-access in sock_hash_free+0x13c/0x280 | Write of size 8 at addr dead000000000122 by task kworker/2:1/95 | | CPU: 2 PID: 95 Comm: kworker/2:1 Not tainted 5.7.0-rc7-02961-ge22c35ab0038-dirty #691 | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014 | Workqueue: events bpf_map_free_deferred | Call Trace: | dump_stack+0x97/0xe0 | ? sock_hash_free+0x13c/0x280 | __kasan_report.cold+0x5/0x40 | ? mark_lock+0xbc1/0xc00 | ? sock_hash_free+0x13c/0x280 | kasan_report+0x38/0x50 | ? sock_hash_free+0x152/0x280 | sock_hash_free+0x13c/0x280 | bpf_map_free_deferred+0xb2/0xd0 | ? bpf_map_charge_finish+0x50/0x50 | ? rcu_read_lock_sched_held+0x81/0xb0 | ? rcu_read_lock_bh_held+0x90/0x90 | process_one_work+0x59a/0xac0 | ? lock_release+0x3b0/0x3b0 | ? pwq_dec_nr_in_flight+0x110/0x110 | ? rwlock_bug.part.0+0x60/0x60 | worker_thread+0x7a/0x680 | ? _raw_spin_unlock_irqrestore+0x4c/0x60 | kthread+0x1cc/0x220 | ? process_one_work+0xac0/0xac0 | ? kthread_create_on_node+0xa0/0xa0 | ret_from_fork+0x24/0x30 | ==================================================================
Fix it by reintroducing spin-lock protected critical section around the code that removes the elements from the bucket on sockhash free.
To do that we also need to defer processing of removed elements, until out of atomic context so that we can unlink the socket from the map when holding the sock lock.
Fixes: 90db6d772f74 ("bpf, sockmap: Remove bucket->lock from sock_{hash|map}_free") Reported-by: Eric Dumazet eric.dumazet@gmail.com Signed-off-by: Jakub Sitnicki jakub@cloudflare.com Signed-off-by: Alexei Starovoitov ast@kernel.org Acked-by: John Fastabend john.fastabend@gmail.com Link: https://lore.kernel.org/bpf/20200607205229.2389672-3-jakub@cloudflare.com Signed-off-by: Sasha Levin sashal@kernel.org --- net/core/sock_map.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-)
diff --git a/net/core/sock_map.c b/net/core/sock_map.c index 7edbf1e924571..050bfac97cfb5 100644 --- a/net/core/sock_map.c +++ b/net/core/sock_map.c @@ -1006,6 +1006,7 @@ static void sock_hash_free(struct bpf_map *map) { struct bpf_htab *htab = container_of(map, struct bpf_htab, map); struct bpf_htab_bucket *bucket; + struct hlist_head unlink_list; struct bpf_htab_elem *elem; struct hlist_node *node; int i; @@ -1017,13 +1018,31 @@ static void sock_hash_free(struct bpf_map *map) synchronize_rcu(); for (i = 0; i < htab->buckets_num; i++) { bucket = sock_hash_select_bucket(htab, i); - hlist_for_each_entry_safe(elem, node, &bucket->head, node) { - hlist_del_rcu(&elem->node); + + /* We are racing with sock_hash_delete_from_link to + * enter the spin-lock critical section. Every socket on + * the list is still linked to sockhash. Since link + * exists, psock exists and holds a ref to socket. That + * lets us to grab a socket ref too. + */ + raw_spin_lock_bh(&bucket->lock); + hlist_for_each_entry(elem, &bucket->head, node) + sock_hold(elem->sk); + hlist_move_list(&bucket->head, &unlink_list); + raw_spin_unlock_bh(&bucket->lock); + + /* Process removed entries out of atomic context to + * block for socket lock before deleting the psock's + * link to sockhash. + */ + hlist_for_each_entry_safe(elem, node, &unlink_list, node) { + hlist_del(&elem->node); lock_sock(elem->sk); rcu_read_lock(); sock_map_unref(elem->sk, elem); rcu_read_unlock(); release_sock(elem->sk); + sock_put(elem->sk); sock_hash_free_elem(htab, elem); } }