io: fix stack allocation when sending of file descriptors

When sending file descriptors over a socket, we have to
allocate a data buffer to hold the FDs in the scmsghdr.
Unfortunately we allocated the buffer on the stack inside
an if () {} block, but called sendmsg() outside the block.
So the stack bytes holding the FDs were liable to be
overwritten with other data. By luck this was not a problem
when sending 1 FD, but if sending 2 or more then it would
fail.

The fix is to simply move the variables outside the nested
'if' block. To keep valgrind quiet we also zero-initialize
the 'control' buffer.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/io/channel-socket.c b/io/channel-socket.c
index eed2ff5bcf..10a5b3136e 100644
--- a/io/channel-socket.c
+++ b/io/channel-socket.c
@@ -493,15 +493,14 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
     QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
     ssize_t ret;
     struct msghdr msg = { NULL, };
+    char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)] = { 0 };
+    size_t fdsize = sizeof(int) * nfds;
+    struct cmsghdr *cmsg;
 
     msg.msg_iov = (struct iovec *)iov;
     msg.msg_iovlen = niov;
 
     if (nfds) {
-        char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)];
-        size_t fdsize = sizeof(int) * nfds;
-        struct cmsghdr *cmsg;
-
         if (nfds > SOCKET_MAX_FDS) {
             error_setg_errno(errp, -EINVAL,
                              "Only %d FDs can be sent, got %zu",