[PATCH v3] vsock: ignore empty child namespace mode writes

[Aldo Ariel Panzardo <qwe.aldo@gmail.com>]

__vsock_net_mode_string() returns success without updating new_mode when
the transfer length is zero. Its caller then reads the uninitialized enum
and may permanently store a stack-derived value in the write-once child
mode.

Return before calling __vsock_net_mode_string() when *lenp is zero so
that the helper is never invoked with nothing to parse and new_mode is
never read uninitialized. This also prevents an empty write from
locking the current mode.

Fixes: eafb64f40ca4 ("vsock: add netns to vsock core")
Cc: stable@vger.kernel.org
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
---

Changes in v3:
  - Move the !*lenp early return before __vsock_net_mode_string()
    instead of after it, as suggested by Stefano Garzarella.
  - Add missing maintainer CCs per get_maintainer.pl.
  - Add Luigi's Reviewed-by.

Changes in v2:
  - Return early instead of initializing new_mode to the current
    value, which would silently consume the write-once transition.

 net/vmw_vsock/af_vsock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 622dbd0467..XXXXXXX 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -2879,6 +2879,9 @@ static int vsock_net_child_mode_string(const struct ctl_table *table, int write,

 	net = container_of(table->data, struct net, vsock.child_ns_mode);

+	if (!*lenp)
+		return 0;
+
 	ret = __vsock_net_mode_string(table, write, buffer, lenp, ppos,
 				      vsock_net_child_mode(net), &new_mode);
 	if (ret)

--
2.43.0