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net/packet: tpacket_rcv: avoid a producer race condition

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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>
This commit is contained in:
Willem de Bruijn 2020-03-13 12:18:09 -04:00 committed by David S. Miller
parent e1f8f78ffe
commit 61fad6816f
2 changed files with 25 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

21
net/packet/af_packet.c
View File

@ -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:

3
net/packet/internal.h
View File

@ -70,7 +70,10 @@ struct packet_ring_buffer {
unsigned int __percpu *pending_refcnt;
union {
unsigned long *rx_owner_map;
struct tpacket_kbdq_core prb_bdqc;
};
};
extern struct mutex fanout_mutex;