Re: [Qemu-devel] [RFC v2 04/12] Add vhost-user-backend

[Daniel P . Berrangé]

On Fri, Jun 01, 2018 at 06:27:41PM +0200, Marc-André Lureau wrote:
> Create a vhost-user-backend object that holds a connection to a
> vhost-user backend and can be referenced from virtio devices that
> support it. See later patches for input & gpu usage.
> 
> A chardev can be specified to communicate with the vhost-user backend,
> ex: -chardev socket,id=char0,path=/tmp/foo.sock -object
> vhost-user-backend,id=vuid,chardev=char0.
> 
> Alternatively, an executable with its arguments may be given as 'cmd'
> property, ex: -object
> vhost-user-backend,id=vui,cmd="./vhost-user-input /dev/input..". The
> executable is then spawn and, by convention, the vhost-user socket is
> passed as fd=3. It may be considered a security breach to allow
> creating processes that may execute arbitrary executables, so this may
> be restricted to some known executables (via signature etc) or
> directory.

Passing a binary and args as a string blob.....

> +static int
> +vhost_user_backend_spawn_cmd(VhostUserBackend *b, int vhostfd, Error **errp)
> +{
> +    int devnull = open("/dev/null", O_RDWR);
> +    pid_t pid;
> +
> +    assert(!b->child);
> +
> +    if (!b->cmd) {
> +        error_setg_errno(errp, errno, "Missing cmd property");
> +        return -1;
> +    }
> +    if (devnull < 0) {
> +        error_setg_errno(errp, errno, "Unable to open /dev/null");
> +        return -1;
> +    }
> +
> +    pid = qemu_fork(errp);
> +    if (pid < 0) {
> +        close(devnull);
> +        return -1;
> +    }
> +
> +    if (pid == 0) { /* child */
> +        int fd, maxfd = sysconf(_SC_OPEN_MAX);
> +
> +        dup2(devnull, STDIN_FILENO);
> +        dup2(devnull, STDOUT_FILENO);
> +        dup2(vhostfd, 3);
> +
> +        signal(SIGINT, SIG_IGN);

Why ignore SIGINT ?  Surely we want this extra process to be killed
someone ctrl-c's the parent QEMU.

> +
> +        for (fd = 4; fd < maxfd; fd++) {
> +            close(fd);
> +        }
> +
> +        execlp("/bin/sh", "sh", "-c", b->cmd, NULL);

...which is then interpreted by the shell is a recipe for security
flaws. There needs to be a way to pass the command + arguments
to QEMU as an argv[] we can directly exec without involving the
shell.

> +        _exit(1);
> +    }
> +
> +    b->child = QIO_CHANNEL(qio_channel_command_new_pid(devnull, devnull, 
> pid));

Overall this method overall duplicates much of the
qio_channel_command_new_argv(), though you do have a few differences.

I'd prefer if we could make qio_channel_command_new_argv more flexible to
handle these extra needs though.

Regards,
Daniel
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