Re: [PATCH net v2] tun: fix ubuf refcount incorrectly on error path
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2020-12-09 14:44:35
Also in:
virtualization
On Wed, Dec 9, 2020 at 8:03 AM wangyunjian [off-list ref] wrote:
From: Yunjian Wang <redacted> After setting callback for ubuf_info of skb, the callback (vhost_net_zerocopy_callback) will be called to decrease the refcount when freeing skb. But when an exception occurs
With exception, you mean if tun_get_user returns an error that propagates to the sendmsg call in vhost handle_tx, correct?
afterwards, the error handling in vhost handle_tx() will try to decrease the same refcount again. This is wrong and fix this by delay copying ubuf_info until we're sure there's no errors.
I think the right approach is to address this in the error paths, rather than complicate the normal datapath. Is it sufficient to suppress the call to vhost_net_ubuf_put in the handle_tx sendmsg error path, given that vhost_zerocopy_callback will be called on kfree_skb? Or alternatively clear the destructor in drop:
quoted hunk ↗ jump to hunk
Fixes: 4477138fa0ae ("tun: properly test for IFF_UP") Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver") Signed-off-by: Yunjian Wang <redacted> --- v2: Updated code, fix by delay copying ubuf_info --- drivers/net/tun.c | 29 +++++++++++++++++++---------- 1 file changed, 19 insertions(+), 10 deletions(-)diff --git a/drivers/net/tun.c b/drivers/net/tun.c index 2dc1988a8973..2ea822328e73 100644 --- a/drivers/net/tun.c +++ b/drivers/net/tun.c@@ -1637,6 +1637,20 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun, return NULL; } +/* copy ubuf_info for callback when skb has no error */ +static inline void tun_copy_ubuf_info(struct sk_buff *skb, bool zerocopy, void *msg_control) +{ + if (zerocopy) { + skb_shinfo(skb)->destructor_arg = msg_control; + skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY; + skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG; + } else if (msg_control) { + struct ubuf_info *uarg = msg_control; + + uarg->callback(uarg, false); + } +} + /* Get packet from user space buffer */ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, void *msg_control, struct iov_iter *from,@@ -1812,16 +1826,6 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, break; } - /* copy skb_ubuf_info for callback when skb has no error */ - if (zerocopy) { - skb_shinfo(skb)->destructor_arg = msg_control; - skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY; - skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG; - } else if (msg_control) { - struct ubuf_info *uarg = msg_control; - uarg->callback(uarg, false); - } - skb_reset_network_header(skb); skb_probe_transport_header(skb); skb_record_rx_queue(skb, tfile->queue_index);@@ -1830,6 +1834,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, struct bpf_prog *xdp_prog; int ret; + tun_copy_ubuf_info(skb, zerocopy, msg_control); local_bh_disable(); rcu_read_lock(); xdp_prog = rcu_dereference(tun->xdp_prog);@@ -1881,6 +1886,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, return -ENOMEM; } + tun_copy_ubuf_info(skb, zerocopy, msg_control); local_bh_disable(); napi_gro_frags(&tfile->napi); local_bh_enable();@@ -1889,6 +1895,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, struct sk_buff_head *queue = &tfile->sk.sk_write_queue; int queue_len; + tun_copy_ubuf_info(skb, zerocopy, msg_control); spin_lock_bh(&queue->lock); __skb_queue_tail(queue, skb); queue_len = skb_queue_len(queue);@@ -1899,8 +1906,10 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile, local_bh_enable(); } else if (!IS_ENABLED(CONFIG_4KSTACKS)) { + tun_copy_ubuf_info(skb, zerocopy, msg_control); tun_rx_batched(tun, tfile, skb, more); } else { + tun_copy_ubuf_info(skb, zerocopy, msg_control); netif_rx_ni(skb); } rcu_read_unlock(); --2.23.0