[PATCH v2] vsock: ignore empty child namespace mode writes

__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 inspecting or storing new_mode when no bytes were
transferred. This also prevents an empty write from locking the current
mode.

Fixes: eafb64f40ca4 ("vsock: add netns to vsock core")
Cc: stable@vger.kernel.org
Signed-off-by: Aldo Ariel Panzardo <redacted>
---
 net/vmw_vsock/af_vsock.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 622dbd0467..816d25b314 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c

@@ -2883,6 +2883,8 @@ static int vsock_net_child_mode_string(const struct ctl_table *table, int write,
 				      vsock_net_child_mode(net), &new_mode);
 	if (ret)
 		return ret;
+	if (!*lenp)
+		return 0;
 
 	if (write) {
 		/* Prevent a "local" namespace from escalating to "global",

-- 
2.43.0