[SRU][PATCH 1/1][aws/groovy] vsock: forward all packets to the host when no H2G is registered

[Kamal Mostafa]

From: Stefano Garzarella <sgarzare at redhat.com>

BugLink: https://bugs.launchpad.net/bugs/1903087

Before commit c0cfa2d8a788 ("vsock: add multi-transports support"),
if a G2H transport was loaded (e.g. virtio transport), every packets
was forwarded to the host, regardless of the destination CID.
The H2G transports implemented until then (vhost-vsock, VMCI) always
responded with an error, if the destination CID was not
VMADDR_CID_HOST.

>From that commit, we are using the remote CID to decide which
transport to use, so packets with remote CID > VMADDR_CID_HOST(2)
are sent only through H2G transport. If no H2G is available, packets
are discarded directly in the guest.

Some use cases (e.g. Nitro Enclaves [1]) rely on the old behaviour
to implement sibling VMs communication, so we restore the old
behavior when no H2G is registered.
It will be up to the host to discard packets if the destination is
not the right one. As it was already implemented before adding
multi-transport support.

Tested with nested QEMU/KVM by me and Nitro Enclaves by Andra.

[1] Documentation/virt/ne_overview.rst

Cc: Jorgen Hansen <jhansen at vmware.com>
Cc: Dexuan Cui <decui at microsoft.com>
Fixes: c0cfa2d8a788 ("vsock: add multi-transports support")
Reported-by: Andra Paraschiv <andraprs at amazon.com>
Tested-by: Andra Paraschiv <andraprs at amazon.com>
Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
Link: https://lore.kernel.org/r/20201112133837.34183-1-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(cherry picked from commit 65b422d9b61ba12c08150784e8012fa1892ad03e)
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 net/vmw_vsock/af_vsock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 6cd0df1c5caf..5fa4b1ac3c96 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -438,7 +438,7 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
 	case SOCK_STREAM:
 		if (vsock_use_local_transport(remote_cid))
 			new_transport = transport_local;
-		else if (remote_cid <= VMADDR_CID_HOST)
+		else if (remote_cid <= VMADDR_CID_HOST || !transport_h2g)
 			new_transport = transport_g2h;
 		else
 			new_transport = transport_h2g;
-- 
2.17.1