[PATCH net-next v4] net: restore the iterator to its original state when an error occurs

[PATCH net-next v4] net: restore the iterator to its original state when an error occurs

[Edward Adam Davis]

In zerocopy_fill_skb_from_iter(), if two copy operations are performed
and the first one succeeds while the second one fails, it returns a
failure but the count in iterator has already been decremented due to
the first successful copy. This ultimately affects the local variable
rest_len in virtio_transport_send_pkt_info(), causing the remaining
count in rest_len to be greater than the actual iterator count. As a
result, packet sending operations continue even when the iterator count
is zero, which further leads to skb->len being 0 and triggers the warning
reported by syzbot [1].

Therefore, if the zerocopy operation fails, we should revert the iterator
to its original state.

The iov_iter_revert() in skb_zerocopy_iter_stream() is no longer needed
and has been removed.

Regarding the judgment condition, I aligned it with the condition in
skb_zerocopy_iter_stream().

[1]
'send_pkt()' returns 0, but 4096 expected
WARNING: net/vmw_vsock/virtio_transport_common.c:430 at virtio_transport_send_pkt_info+0xd1e/0xef0 net/vmw_vsock/virtio_transport_common.c:428, CPU#1: syz.0.17/5986
Call Trace:
 virtio_transport_stream_enqueue net/vmw_vsock/virtio_transport_common.c:1113 [inline]
 virtio_transport_seqpacket_enqueue+0x143/0x1c0 net/vmw_vsock/virtio_transport_common.c:841
 vsock_connectible_sendmsg+0xabf/0x1040 net/vmw_vsock/af_vsock.c:2158
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg+0x21c/0x270 net/socket.c:746

Reported-by: syzbot+28e5f3d207b14bae122a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=28e5f3d207b14bae122a
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
v4:
  - add comments for new condition
v3: https://lore.kernel.org/all/tencent_3C86DFD37A0374496263BE24483777D76305@qq.com/
  - fix test tcp_zerocopy_maxfrags timeout
v2: https://lore.kernel.org/all/tencent_BA768766163C533724966E36344AAE754709@qq.com/
  - Remove iov_iter_revert() in skb_zerocopy_iter_stream()
v1: https://lore.kernel.org/all/tencent_387517772566B03DBD365896C036264AA809@qq.com/

 net/core/datagram.c | 9 ++++++++-
 net/core/skbuff.c   | 1 -
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/core/datagram.c b/net/core/datagram.c
index c285c6465923..c5f2f1b8786b 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -749,8 +749,12 @@ int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,
 			    struct net_devmem_dmabuf_binding *binding)
 {
 	unsigned long orig_size = skb->truesize;
+	struct iov_iter_state state;
 	unsigned long truesize;
-	int ret;
+	int ret, orig_len;
+
+	iov_iter_save_state(from, &state);
+	orig_len = skb->len;
 
 	if (msg && msg->msg_ubuf && msg->sg_from_iter)
 		ret = msg->sg_from_iter(skb, from, length);
@@ -759,6 +763,9 @@ int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk,
 	else
 		ret = zerocopy_fill_skb_from_iter(skb, from, length);
 
+	if (ret == -EFAULT || (ret == -EMSGSIZE && skb->len == orig_len))
+		iov_iter_restore(from, &state);
+
 	truesize = skb->truesize - orig_size;
 	if (sk && sk->sk_type == SOCK_STREAM) {
 		sk_wmem_queued_add(sk, truesize);
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a00808f7be6a..7b8836f668b7 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1908,7 +1908,6 @@ int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
 		struct sock *save_sk = skb->sk;
 
 		/* Streams do not free skb on error. Reset to prev state. */
-		iov_iter_revert(&msg->msg_iter, skb->len - orig_len);
 		skb->sk = sk;
 		___pskb_trim(skb, orig_len);
 		skb->sk = save_sk;
-- 
2.43.0

Re: [PATCH net-next v4] net: restore the iterator to its original state when an error occurs

[Jakub Kicinski]

On Thu, 11 Dec 2025 14:57:08 +0800 Edward Adam Davis wrote:
> In zerocopy_fill_skb_from_iter(), if two copy operations are performed
> and the first one succeeds while the second one fails, it returns a
> failure but the count in iterator has already been decremented due to
> the first successful copy. This ultimately affects the local variable
> rest_len in virtio_transport_send_pkt_info(), causing the remaining
> count in rest_len to be greater than the actual iterator count. As a
> result, packet sending operations continue even when the iterator count
> is zero, which further leads to skb->len being 0 and triggers the warning
> reported by syzbot [1].

Please address the feedback from previous revision and when you repost
use net as the subject tag.
-- 
pw-bot: cr

Re: [PATCH net-next v4] net: restore the iterator to its original state when an error occurs

[Edward Adam Davis]

On Sat, 13 Dec 2025 08:37:17 +0900, Jakub Kicinski wrote:
> > In zerocopy_fill_skb_from_iter(), if two copy operations are performed
> > and the first one succeeds while the second one fails, it returns a
> > failure but the count in iterator has already been decremented due to
> > the first successful copy. This ultimately affects the local variable
> > rest_len in virtio_transport_send_pkt_info(), causing the remaining
> > count in rest_len to be greater than the actual iterator count. As a
> > result, packet sending operations continue even when the iterator count
> > is zero, which further leads to skb->len being 0 and triggers the warning
> > reported by syzbot [1].
> 
> Please address the feedback from previous revision and when you repost
> use net as the subject tag.
I have added the following explanation in the comments:
Regarding the judgment condition, I aligned it with the condition in
skb_zerocopy_iter_stream().

syzbot reported the issue in the linux-next repository, and I also
tested and created the patch using the linux-next source code repository.
Therefore, I added the subject net-next tag.
If you think adding the net subject tag directly is acceptable, I will
adjust it in the next version of the patch.