virtio_net: use u64_stats_t infra to avoid data-races

syzbot reported a data-race in virtnet_poll / virtnet_stats [1]

u64_stats_t infra has very nice accessors that must be used
to avoid potential load-store tearing.

[1]
BUG: KCSAN: data-race in virtnet_poll / virtnet_stats

read-write to 0xffff88810271b1a0 of 8 bytes by interrupt on cpu 0:
virtnet_receive drivers/net/virtio_net.c:2102 [inline]
virtnet_poll+0x6c8/0xb40 drivers/net/virtio_net.c:2148
__napi_poll+0x60/0x3b0 net/core/dev.c:6527
napi_poll net/core/dev.c:6594 [inline]
net_rx_action+0x32b/0x750 net/core/dev.c:6727
__do_softirq+0xc1/0x265 kernel/softirq.c:553
invoke_softirq kernel/softirq.c:427 [inline]
__irq_exit_rcu kernel/softirq.c:632 [inline]
irq_exit_rcu+0x3b/0x90 kernel/softirq.c:644
common_interrupt+0x7f/0x90 arch/x86/kernel/irq.c:247
asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:636
__sanitizer_cov_trace_const_cmp8+0x0/0x80 kernel/kcov.c:306
jbd2_write_access_granted fs/jbd2/transaction.c:1174 [inline]
jbd2_journal_get_write_access+0x94/0x1c0 fs/jbd2/transaction.c:1239
__ext4_journal_get_write_access+0x154/0x3f0 fs/ext4/ext4_jbd2.c:241
ext4_reserve_inode_write+0x14e/0x200 fs/ext4/inode.c:5745
__ext4_mark_inode_dirty+0x8e/0x440 fs/ext4/inode.c:5919
ext4_evict_inode+0xaf0/0xdc0 fs/ext4/inode.c:299
evict+0x1aa/0x410 fs/inode.c:664
iput_final fs/inode.c:1775 [inline]
iput+0x42c/0x5b0 fs/inode.c:1801
do_unlinkat+0x2b9/0x4f0 fs/namei.c:4405
__do_sys_unlink fs/namei.c:4446 [inline]
__se_sys_unlink fs/namei.c:4444 [inline]
__x64_sys_unlink+0x30/0x40 fs/namei.c:4444
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffff88810271b1a0 of 8 bytes by task 2814 on cpu 1:
virtnet_stats+0x1b3/0x340 drivers/net/virtio_net.c:2564
dev_get_stats+0x6d/0x860 net/core/dev.c:10511
rtnl_fill_stats+0x45/0x320 net/core/rtnetlink.c:1261
rtnl_fill_ifinfo+0xd0e/0x1120 net/core/rtnetlink.c:1867
rtnl_dump_ifinfo+0x7f9/0xc20 net/core/rtnetlink.c:2240
netlink_dump+0x390/0x720 net/netlink/af_netlink.c:2266
netlink_recvmsg+0x425/0x780 net/netlink/af_netlink.c:1992
sock_recvmsg_nosec net/socket.c:1027 [inline]
sock_recvmsg net/socket.c:1049 [inline]
____sys_recvmsg+0x156/0x310 net/socket.c:2760
___sys_recvmsg net/socket.c:2802 [inline]
__sys_recvmsg+0x1ea/0x270 net/socket.c:2832
__do_sys_recvmsg net/socket.c:2842 [inline]
__se_sys_recvmsg net/socket.c:2839 [inline]
__x64_sys_recvmsg+0x46/0x50 net/socket.c:2839
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x000000000045c334 -> 0x000000000045c376

Fixes: 3fa2a1df ("virtio-net: per cpu 64 bit stats (v2)")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -81,24 +81,24 @@ struct virtnet_stat_desc {
 
 struct virtnet_sq_stats {
 	struct u64_stats_sync syncp;
-	u64 packets;
-	u64 bytes;
-	u64 xdp_tx;
-	u64 xdp_tx_drops;
-	u64 kicks;
-	u64 tx_timeouts;
+	u64_stats_t packets;
+	u64_stats_t bytes;
+	u64_stats_t xdp_tx;
+	u64_stats_t xdp_tx_drops;
+	u64_stats_t kicks;
+	u64_stats_t tx_timeouts;
 };
 
 struct virtnet_rq_stats {
 	struct u64_stats_sync syncp;
-	u64 packets;
-	u64 bytes;
-	u64 drops;
-	u64 xdp_packets;
-	u64 xdp_tx;
-	u64 xdp_redirects;
-	u64 xdp_drops;
-	u64 kicks;
+	u64_stats_t packets;
+	u64_stats_t bytes;
+	u64_stats_t drops;
+	u64_stats_t xdp_packets;
+	u64_stats_t xdp_tx;
+	u64_stats_t xdp_redirects;
+	u64_stats_t xdp_drops;
+	u64_stats_t kicks;
 };
 
 #define VIRTNET_SQ_STAT(m)	offsetof(struct virtnet_sq_stats, m)
@@ -775,8 +775,8 @@ static void free_old_xmit_skbs(struct send_queue *sq, bool in_napi)
 		return;
 
 	u64_stats_update_begin(&sq->stats.syncp);
-	sq->stats.bytes += bytes;
-	sq->stats.packets += packets;
+	u64_stats_add(&sq->stats.bytes, bytes);
+	u64_stats_add(&sq->stats.packets, packets);
 	u64_stats_update_end(&sq->stats.syncp);
 }
 
@@ -975,11 +975,11 @@ static int virtnet_xdp_xmit(struct net_device *dev,
 	}
 out:
 	u64_stats_update_begin(&sq->stats.syncp);
-	sq->stats.bytes += bytes;
-	sq->stats.packets += packets;
-	sq->stats.xdp_tx += n;
-	sq->stats.xdp_tx_drops += n - nxmit;
-	sq->stats.kicks += kicks;
+	u64_stats_add(&sq->stats.bytes, bytes);
+	u64_stats_add(&sq->stats.packets, packets);
+	u64_stats_add(&sq->stats.xdp_tx, n);
+	u64_stats_add(&sq->stats.xdp_tx_drops, n - nxmit);
+	u64_stats_add(&sq->stats.kicks, kicks);
 	u64_stats_update_end(&sq->stats.syncp);
 
 	virtnet_xdp_put_sq(vi, sq);
@@ -1011,14 +1011,14 @@ static int virtnet_xdp_handler(struct bpf_prog *xdp_prog, struct xdp_buff *xdp,
 	u32 act;
 
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
-	stats->xdp_packets++;
+	u64_stats_inc(&stats->xdp_packets);
 
 	switch (act) {
 	case XDP_PASS:
 		return act;
 
 	case XDP_TX:
-		stats->xdp_tx++;
+		u64_stats_inc(&stats->xdp_tx);
 		xdpf = xdp_convert_buff_to_frame(xdp);
 		if (unlikely(!xdpf)) {
 			netdev_dbg(dev, "convert buff to frame failed for xdp\n");
@@ -1036,7 +1036,7 @@ static int virtnet_xdp_handler(struct bpf_prog *xdp_prog, struct xdp_buff *xdp,
 		return act;
 
 	case XDP_REDIRECT:
-		stats->xdp_redirects++;
+		u64_stats_inc(&stats->xdp_redirects);
 		err = xdp_do_redirect(dev, xdp, xdp_prog);
 		if (err)
 			return XDP_DROP;
@@ -1232,9 +1232,9 @@ static struct sk_buff *receive_small_xdp(struct net_device *dev,
 	return skb;
 
 err_xdp:
-	stats->xdp_drops++;
+	u64_stats_inc(&stats->xdp_drops);
 err:
-	stats->drops++;
+	u64_stats_inc(&stats->drops);
 	put_page(page);
 xdp_xmit:
 	return NULL;
@@ -1253,7 +1253,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
 	struct sk_buff *skb;
 
 	len -= vi->hdr_len;
-	stats->bytes += len;
+	u64_stats_add(&stats->bytes, len);
 
 	if (unlikely(len > GOOD_PACKET_LEN)) {
 		pr_debug("%s: rx error: len %u exceeds max size %d\n",
@@ -1282,7 +1282,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
 		return skb;
 
 err:
-	stats->drops++;
+	u64_stats_inc(&stats->drops);
 	put_page(page);
 	return NULL;
 }
@@ -1298,14 +1298,14 @@ static struct sk_buff *receive_big(struct net_device *dev,
 	struct sk_buff *skb =
 		page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
 
-	stats->bytes += len - vi->hdr_len;
+	u64_stats_add(&stats->bytes, len - vi->hdr_len);
 	if (unlikely(!skb))
 		goto err;
 
 	return skb;
 
 err:
-	stats->drops++;
+	u64_stats_inc(&stats->drops);
 	give_pages(rq, page);
 	return NULL;
 }
@@ -1326,7 +1326,7 @@ static void mergeable_buf_free(struct receive_queue *rq, int num_buf,
 			DEV_STATS_INC(dev, rx_length_errors);
 			break;
 		}
-		stats->bytes += len;
+		u64_stats_add(&stats->bytes, len);
 		page = virt_to_head_page(buf);
 		put_page(page);
 	}
@@ -1436,7 +1436,7 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
 			goto err;
 		}
 
-		stats->bytes += len;
+		u64_stats_add(&stats->bytes, len);
 		page = virt_to_head_page(buf);
 		offset = buf - page_address(page);
 
@@ -1600,8 +1600,8 @@ static struct sk_buff *receive_mergeable_xdp(struct net_device *dev,
 	put_page(page);
 	mergeable_buf_free(rq, num_buf, dev, stats);
 
-	stats->xdp_drops++;
-	stats->drops++;
+	u64_stats_inc(&stats->xdp_drops);
+	u64_stats_inc(&stats->drops);
 	return NULL;
 }
 
@@ -1625,7 +1625,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 	unsigned int room = SKB_DATA_ALIGN(headroom + tailroom);
 
 	head_skb = NULL;
-	stats->bytes += len - vi->hdr_len;
+	u64_stats_add(&stats->bytes, len - vi->hdr_len);
 
 	if (unlikely(len > truesize - room)) {
 		pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
@@ -1666,7 +1666,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			goto err_buf;
 		}
 
-		stats->bytes += len;
+		u64_stats_add(&stats->bytes, len);
 		page = virt_to_head_page(buf);
 
 		truesize = mergeable_ctx_to_truesize(ctx);
@@ -1718,7 +1718,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 	mergeable_buf_free(rq, num_buf, dev, stats);
 
 err_buf:
-	stats->drops++;
+	u64_stats_inc(&stats->drops);
 	dev_kfree_skb(head_skb);
 	return NULL;
 }
@@ -1985,7 +1985,7 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
 		unsigned long flags;
 
 		flags = u64_stats_update_begin_irqsave(&rq->stats.syncp);
-		rq->stats.kicks++;
+		u64_stats_inc(&rq->stats.kicks);
 		u64_stats_update_end_irqrestore(&rq->stats.syncp, flags);
 	}
 
@@ -2065,22 +2065,23 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
 	struct virtnet_info *vi = rq->vq->vdev->priv;
 	struct virtnet_rq_stats stats = {};
 	unsigned int len;
+	int packets = 0;
 	void *buf;
 	int i;
 
 	if (!vi->big_packets || vi->mergeable_rx_bufs) {
 		void *ctx;
 
-		while (stats.packets < budget &&
+		while (packets < budget &&
 		       (buf = virtnet_rq_get_buf(rq, &len, &ctx))) {
 			receive_buf(vi, rq, buf, len, ctx, xdp_xmit, &stats);
-			stats.packets++;
+			packets++;
 		}
 	} else {
-		while (stats.packets < budget &&
+		while (packets < budget &&
 		       (buf = virtnet_rq_get_buf(rq, &len, NULL)) != NULL) {
 			receive_buf(vi, rq, buf, len, NULL, xdp_xmit, &stats);
-			stats.packets++;
+			packets++;
 		}
 	}
 
@@ -2093,17 +2094,19 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
 		}
 	}
 
+	u64_stats_set(&stats.packets, packets);
 	u64_stats_update_begin(&rq->stats.syncp);
 	for (i = 0; i < VIRTNET_RQ_STATS_LEN; i++) {
 		size_t offset = virtnet_rq_stats_desc[i].offset;
-		u64 *item;
+		u64_stats_t *item, *src;
 
-		item = (u64 *)((u8 *)&rq->stats + offset);
-		*item += *(u64 *)((u8 *)&stats + offset);
+		item = (u64_stats_t *)((u8 *)&rq->stats + offset);
+		src = (u64_stats_t *)((u8 *)&stats + offset);
+		u64_stats_add(item, u64_stats_read(src));
 	}
 	u64_stats_update_end(&rq->stats.syncp);
 
-	return stats.packets;
+	return packets;
 }
 
 static void virtnet_poll_cleantx(struct receive_queue *rq)
@@ -2158,7 +2161,7 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 		sq = virtnet_xdp_get_sq(vi);
 		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
 			u64_stats_update_begin(&sq->stats.syncp);
-			sq->stats.kicks++;
+			u64_stats_inc(&sq->stats.kicks);
 			u64_stats_update_end(&sq->stats.syncp);
 		}
 		virtnet_xdp_put_sq(vi, sq);
@@ -2370,7 +2373,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (kick || netif_xmit_stopped(txq)) {
 		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
 			u64_stats_update_begin(&sq->stats.syncp);
-			sq->stats.kicks++;
+			u64_stats_inc(&sq->stats.kicks);
 			u64_stats_update_end(&sq->stats.syncp);
 		}
 	}
@@ -2553,16 +2556,16 @@ static void virtnet_stats(struct net_device *dev,
 
 		do {
 			start = u64_stats_fetch_begin(&sq->stats.syncp);
-			tpackets = sq->stats.packets;
-			tbytes   = sq->stats.bytes;
-			terrors  = sq->stats.tx_timeouts;
+			tpackets = u64_stats_read(&sq->stats.packets);
+			tbytes   = u64_stats_read(&sq->stats.bytes);
+			terrors  = u64_stats_read(&sq->stats.tx_timeouts);
 		} while (u64_stats_fetch_retry(&sq->stats.syncp, start));
 
 		do {
 			start = u64_stats_fetch_begin(&rq->stats.syncp);
-			rpackets = rq->stats.packets;
-			rbytes   = rq->stats.bytes;
-			rdrops   = rq->stats.drops;
+			rpackets = u64_stats_read(&rq->stats.packets);
+			rbytes   = u64_stats_read(&rq->stats.bytes);
+			rdrops   = u64_stats_read(&rq->stats.drops);
 		} while (u64_stats_fetch_retry(&rq->stats.syncp, start));
 
 		tot->rx_packets += rpackets;
@@ -3191,17 +3194,19 @@ static void virtnet_get_ethtool_stats(struct net_device *dev,
 	struct virtnet_info *vi = netdev_priv(dev);
 	unsigned int idx = 0, start, i, j;
 	const u8 *stats_base;
+	const u64_stats_t *p;
 	size_t offset;
 
 	for (i = 0; i < vi->curr_queue_pairs; i++) {
 		struct receive_queue *rq = &vi->rq[i];
 
-		stats_base = (u8 *)&rq->stats;
+		stats_base = (const u8 *)&rq->stats;
 		do {
 			start = u64_stats_fetch_begin(&rq->stats.syncp);
 			for (j = 0; j < VIRTNET_RQ_STATS_LEN; j++) {
 				offset = virtnet_rq_stats_desc[j].offset;
-				data[idx + j] = *(u64 *)(stats_base + offset);
+				p = (const u64_stats_t *)(stats_base + offset);
+				data[idx + j] = u64_stats_read(p);
 			}
 		} while (u64_stats_fetch_retry(&rq->stats.syncp, start));
 		idx += VIRTNET_RQ_STATS_LEN;
@@ -3210,12 +3215,13 @@ static void virtnet_get_ethtool_stats(struct net_device *dev,
 	for (i = 0; i < vi->curr_queue_pairs; i++) {
 		struct send_queue *sq = &vi->sq[i];
 
-		stats_base = (u8 *)&sq->stats;
+		stats_base = (const u8 *)&sq->stats;
 		do {
 			start = u64_stats_fetch_begin(&sq->stats.syncp);
 			for (j = 0; j < VIRTNET_SQ_STATS_LEN; j++) {
 				offset = virtnet_sq_stats_desc[j].offset;
-				data[idx + j] = *(u64 *)(stats_base + offset);
+				p = (const u64_stats_t *)(stats_base + offset);
+				data[idx + j] = u64_stats_read(p);
 			}
 		} while (u64_stats_fetch_retry(&sq->stats.syncp, start));
 		idx += VIRTNET_SQ_STATS_LEN;
@@ -3898,7 +3904,7 @@ static void virtnet_tx_timeout(struct net_device *dev, unsigned int txqueue)
 	struct netdev_queue *txq = netdev_get_tx_queue(dev, txqueue);
 
 	u64_stats_update_begin(&sq->stats.syncp);
-	sq->stats.tx_timeouts++;
+	u64_stats_inc(&sq->stats.tx_timeouts);
 	u64_stats_update_end(&sq->stats.syncp);
 
 	netdev_err(dev, "TX timeout on queue: %u, sq: %s, vq: 0x%x, name: %s, %u usecs ago\n",