[PATCH net v2 0/2] vsock/virtio: fix msg_iter desync on transmission failure

[PATCH net v2 0/2] vsock/virtio: fix msg_iter desync on transmission failure

[Octavian Purdila]

This series fixes a msg_iter desync issue in the virtio vsock transport
that can lead to warnings and eventual -ENOMEM under specific failure
scenarios (e.g. partial GUP failure during MSG_ZEROCOPY transmission).

To fix this, we need to restore the msg_iter state on transmission failure.
However, since virtio vsock transport can be built as a module, we first
need to export iov_iter_restore.

Patch 1 exports iov_iter_restore.
Patch 2 implements the msg_iter restoration in virtio vsock.

Changes in v2:
- Use iov_iter_savestate()/iov_iter_restore() (Stefano)
- Use a single restore point (Stefano)
- Reverse xmas tree (Stefano)
- Added comments in the code (Stefano)

v1: https://lore.kernel.org/all/20260609004809.1285028-1-tavip@google.com/

Octavian Purdila (2):
  iov_iter: export iov_iter_restore
  vsock/virtio: restore msg_iter on transmission failure

 lib/iov_iter.c                          |  1 +
 net/vmw_vsock/virtio_transport_common.c | 13 +++++++++++++
 2 files changed, 14 insertions(+)

-- 
2.54.0.1136.gdb2ca164c4-goog

[PATCH net v2 1/2] iov_iter: export iov_iter_restore

[Octavian Purdila]

Export iov_iter_restore so that it can be used by modules.

This is needed by the virtio vsock transport (which can be built as a
module) to restore the msg_iter state when transmission fails.

Signed-off-by: Octavian Purdila <tavip@google.com>
---
 lib/iov_iter.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 243662af1af73..067e745f9ef53 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1491,6 +1491,7 @@ void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state)
 		i->__iov -= state->nr_segs - i->nr_segs;
 	i->nr_segs = state->nr_segs;
 }
+EXPORT_SYMBOL(iov_iter_restore);
 
 /*
  * Extract a list of contiguous pages from an ITER_FOLIOQ iterator.  This does
-- 
2.54.0.1136.gdb2ca164c4-goog

[PATCH net v2 2/2] vsock/virtio: restore msg_iter on transmission failure

[Octavian Purdila]

When transmission fails in virtio_transport_send_pkt_info, the msg_iter
might have been partially advanced. If we don't restore it, the next
attempt to send data will use an incorrect iterator state, leading to
desync and warnings like "send_pkt() returns 0, but X expected".

Specifically, this can happen in the following scenario, triggered by
the syzkaller repro:

1. A write-only VMA (PROT_WRITE only) is partially populated by a
   prior TUN write that failed with -EIO but still faulted in some
   pages).
2. A vsock sendmmsg call with MSG_ZEROCOPY requests transmission of a
   buffer from this VMA.
3. The first packet (64KB) is sent successfully because the pages are
   populated.
4. The second packet allocation fails because GUP fast pins the first page
   but GUP slow fails on the next unpopulated page due to PROT_WRITE-only
   permissions.
5. The iterator is advanced by the partially successful GUP (68KB total
   advanced: 64KB from first packet + 4KB from second), but the send loop
   breaks and only reports 64KB sent. This creates a 4KB desync.
6. The next retry starts with a non-zero iov_offset, disabling zerocopy
   and falling back to copy mode.
7. In copy mode, the transmission succeeds for the next packets but
   exhausts the iterator early because of the desync.
8. The final retry sees an empty iterator but zerocopy is re-enabled
   (offset resets). It attempts to send the remaining bytes with zerocopy
   but pins 0 pages, creating an empty packet.
9. The transport sends the empty packet, triggering the warning because
   the returned bytes (header only) do not match the expected payload size.
10. The loop continues to spin, allocating ubuf_info each time, eventually
    exhausting sysctl_optmem_max and returning -ENOMEM to userspace.

Restore msg_iter to its original state before the packet allocation
and transmission attempt if they fail.

Fixes: e0718bd82e27 ("vsock: enable setting SO_ZEROCOPY")
Reported-by: syzbot+28e5f3d207b14bae122a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=28e5f3d207b14bae122a
Assisted-by: gemini:gemini-3.1-pro
Signed-off-by: Octavian Purdila <tavip@google.com>
---
 net/vmw_vsock/virtio_transport_common.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index b10666937c490..2baa5a6ebd750 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -295,6 +295,7 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
 	u32 max_skb_len = VIRTIO_VSOCK_MAX_PKT_BUF_SIZE;
 	u32 src_cid, src_port, dst_cid, dst_port;
 	const struct virtio_transport *t_ops;
+	struct iov_iter_state msg_iter_state;
 	struct virtio_vsock_sock *vvs;
 	struct ubuf_info *uarg = NULL;
 	u32 pkt_len = info->pkt_len;
@@ -368,8 +369,17 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
 		struct sk_buff *skb;
 		size_t skb_len;
 
+		/* Save iterator state in case allocation or transmission fails
+		 * so we can restore it and retry.
+		 */
+		if (info->msg)
+			iov_iter_save_state(&info->msg->msg_iter, &msg_iter_state);
+
 		skb_len = min(max_skb_len, rest_len);
 
+		/* Note: virtio_transport_alloc_skb() can advance info->msg->msg_iter
+		 * even if it fails (e.g. partial GUP success).
+		 */
 		skb = virtio_transport_alloc_skb(info, skb_len, can_zcopy,
 						 uarg,
 						 src_cid, src_port,
@@ -399,6 +409,9 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
 			break;
 	} while (rest_len);
 
+	if (info->msg && ret < 0)
+		iov_iter_restore(&info->msg->msg_iter, &msg_iter_state);
+
 	virtio_transport_put_credit(vvs, rest_len);
 
 	/* msg_zerocopy_realloc() initializes the ubuf_info refcnt to 1.
-- 
2.54.0.1136.gdb2ca164c4-goog