diff options
author | Willem de Bruijn <willemb@google.com> | 2020-03-13 19:18:09 +0300 |
---|---|---|
committer | David S. Miller <davem@davemloft.net> | 2020-03-15 10:25:25 +0300 |
commit | 61fad6816fc10fb8793a925d5c1256d1c3db0cd2 (patch) | |
tree | 25c959f9eeaf0a19a1d0e578f34a07604268437b /net/packet/af_packet.c | |
parent | e1f8f78ffe9854308b9e12a73ebe4e909074fc33 (diff) | |
download | linux-61fad6816fc10fb8793a925d5c1256d1c3db0cd2.tar.xz |
net/packet: tpacket_rcv: avoid a producer race condition
PACKET_RX_RING can cause multiple writers to access the same slot if a
fast writer wraps the ring while a slow writer is still copying. This
is particularly likely with few, large, slots (e.g., GSO packets).
Synchronize kernel thread ownership of rx ring slots with a bitmap.
Writers acquire a slot race-free by testing tp_status TP_STATUS_KERNEL
while holding the sk receive queue lock. They release this lock before
copying and set tp_status to TP_STATUS_USER to release to userspace
when done. During copying, another writer may take the lock, also see
TP_STATUS_KERNEL, and start writing to the same slot.
Introduce a new rx_owner_map bitmap with a bit per slot. To acquire a
slot, test and set with the lock held. To release race-free, update
tp_status and owner bit as a transaction, so take the lock again.
This is the one of a variety of discussed options (see Link below):
* instead of a shadow ring, embed the data in the slot itself, such as
in tp_padding. But any test for this field may match a value left by
userspace, causing deadlock.
* avoid the lock on release. This leaves a small race if releasing the
shadow slot before setting TP_STATUS_USER. The below reproducer showed
that this race is not academic. If releasing the slot after tp_status,
the race is more subtle. See the first link for details.
* add a new tp_status TP_KERNEL_OWNED to avoid the transactional store
of two fields. But, legacy applications may interpret all non-zero
tp_status as owned by the user. As libpcap does. So this is possible
only opt-in by newer processes. It can be added as an optional mode.
* embed the struct at the tail of pg_vec to avoid extra allocation.
The implementation proved no less complex than a separate field.
The additional locking cost on release adds contention, no different
than scaling on multicore or multiqueue h/w. In practice, below
reproducer nor small packet tcpdump showed a noticeable change in
perf report in cycles spent in spinlock. Where contention is
problematic, packet sockets support mitigation through PACKET_FANOUT.
And we can consider adding opt-in state TP_KERNEL_OWNED.
Easy to reproduce by running multiple netperf or similar TCP_STREAM
flows concurrently with `tcpdump -B 129 -n greater 60000`.
Based on an earlier patchset by Jon Rosen. See links below.
I believe this issue goes back to the introduction of tpacket_rcv,
which predates git history.
Link: https://www.mail-archive.com/netdev@vger.kernel.org/msg237222.html
Suggested-by: Jon Rosen <jrosen@cisco.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jon Rosen <jrosen@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/packet/af_packet.c')
-rw-r--r-- | net/packet/af_packet.c | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c index e5b0986215d2..29bd405adbbd 100644 --- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c @@ -2173,6 +2173,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev, struct timespec64 ts; __u32 ts_status; bool is_drop_n_account = false; + unsigned int slot_id = 0; bool do_vnet = false; /* struct tpacket{2,3}_hdr is aligned to a multiple of TPACKET_ALIGNMENT. @@ -2275,6 +2276,13 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev, if (!h.raw) goto drop_n_account; + if (po->tp_version <= TPACKET_V2) { + slot_id = po->rx_ring.head; + if (test_bit(slot_id, po->rx_ring.rx_owner_map)) + goto drop_n_account; + __set_bit(slot_id, po->rx_ring.rx_owner_map); + } + if (do_vnet && virtio_net_hdr_from_skb(skb, h.raw + macoff - sizeof(struct virtio_net_hdr), @@ -2380,7 +2388,10 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev, #endif if (po->tp_version <= TPACKET_V2) { + spin_lock(&sk->sk_receive_queue.lock); __packet_set_status(po, h.raw, status); + __clear_bit(slot_id, po->rx_ring.rx_owner_map); + spin_unlock(&sk->sk_receive_queue.lock); sk->sk_data_ready(sk); } else { prb_clear_blk_fill_status(&po->rx_ring); @@ -4277,6 +4288,7 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u, { struct pgv *pg_vec = NULL; struct packet_sock *po = pkt_sk(sk); + unsigned long *rx_owner_map = NULL; int was_running, order = 0; struct packet_ring_buffer *rb; struct sk_buff_head *rb_queue; @@ -4362,6 +4374,12 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u, } break; default: + if (!tx_ring) { + rx_owner_map = bitmap_alloc(req->tp_frame_nr, + GFP_KERNEL | __GFP_NOWARN | __GFP_ZERO); + if (!rx_owner_map) + goto out_free_pg_vec; + } break; } } @@ -4391,6 +4409,8 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u, err = 0; spin_lock_bh(&rb_queue->lock); swap(rb->pg_vec, pg_vec); + if (po->tp_version <= TPACKET_V2) + swap(rb->rx_owner_map, rx_owner_map); rb->frame_max = (req->tp_frame_nr - 1); rb->head = 0; rb->frame_size = req->tp_frame_size; @@ -4422,6 +4442,7 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u, } out_free_pg_vec: + bitmap_free(rx_owner_map); if (pg_vec) free_pg_vec(pg_vec, order, req->tp_block_nr); out: |