virtio_net: sync RX buffer before reading the header

virtio_net: sync RX buffer before reading the header

receive_buf() reads the virtio header through buf before
page_pool_dma_sync_for_cpu() runs in receive_small() or
receive_mergeable(). The header buffer is thus unsynchronized at the
point where flags and, for mergeable buffers, num_buffers are consumed.

Omar Elghoul reported that on s390x Secure Execution this showed up as
greatly reduced virtio-net performance together with "bad gso" and
"bad csum" messages in dmesg. This is because with SE sync actually
copies data, so the header is uninitialized.

Move the sync into receive_buf() so the
header is synchronized before any access through buf.

Tool use: Cursor with GPT-5.4 drafted the initial code move from prompt:
"in drivers/net/virtio_net.c, move page_pool_dma_sync_for_cpu on receive
path to before memory is accessed through buf".
The result and the commit log were reviewed and edited manually.

Fixes: 24fbd3967f3f ("virtio_net: add page_pool support for buffer allocation")
Reported-by: Omar Elghoul <oelghoul@linux.ibm.com>
Tested-by: Srikanth Aithal <sraithal@amd.com>
Tested-by: Omar Elghoul <oelghoul@linux.ibm.com>
Link: https://lore.kernel.org/r/20260323150136.14452-1-oelghoul@linux.ibm.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Vishwanath Seshagiri <vishs@meta.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/f4caa9be9e5addae7851c012cab0a733be7f0974.1774365273.git.mst@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>