On Sat, Jun 13, 2026 at 12:09:53AM +0000, Octavian Purdila wrote:
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: [email protected] Closes: https://syzkaller.appspot.com/bug?extid=28e5f3d207b14bae122a Assisted-by: gemini:gemini-3.1-pro Signed-off-by: Octavian Purdila <[email protected]> --- net/vmw_vsock/virtio_transport_common.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
Thanks, looks much better to me now! Reviewed-by: Stefano Garzarella <[email protected]>
