Re: [Qemu-devel] [PULL 22/33] sockets: improve error reporting if UNIX socket path is too long

[Peter Maydell]

On 1 June 2017 at 13:41, Paolo Bonzini <address@hidden> wrote:
> From: "Daniel P. Berrange" <address@hidden>
>
> The 'struct sockaddr_un' only allows 108 bytes for the socket
> path.
>
> If the user supplies a path, QEMU uses snprintf() to silently
> truncate it when too long. This is undesirable because the user
> will then be unable to connect to the path they asked for.
>
> If the user doesn't supply a path, QEMU builds one based on
> TMPDIR, but if that leads to an overlong path, it mistakenly
> uses error_setg_errno() with a stale errno value, because
> snprintf() does not set errno on truncation.
>
> In solving this the code needed some refactoring to ensure we
> don't pass 'un.sun_path' directly to any APIs which expect
> NUL-terminated strings, because the path is not required to
> be terminated.
>
> Signed-off-by: Daniel P. Berrange <address@hidden>
> Message-Id: <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  util/qemu-sockets.c | 68 
> ++++++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 46 insertions(+), 22 deletions(-)

It looks like we missed a case where we should have changed
an un.sun_path usage to something else:

> @@ -873,24 +877,25 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
>           * to unlink first and thus re-open the race window.  The
>           * worst case possible is bind() failing, i.e. a DoS attack.
>           */
> -        fd = mkstemp(un.sun_path);
> +        fd = mkstemp(pathbuf);
>          if (fd < 0) {
>              error_setg_errno(errp, errno,
> -                             "Failed to make a temporary socket name in %s", 
> tmpdir);
> +                             "Failed to make a temporary socket %s", 
> pathbuf);
>              goto err;
>          }
>          close(fd);
> -        if (update_addr) {
> -            g_free(saddr->path);
> -            saddr->path = g_strdup(un.sun_path);
> -        }
>      }
>
> -    if (unlink(un.sun_path) < 0 && errno != ENOENT) {
> +    if (unlink(path) < 0 && errno != ENOENT) {
>          error_setg_errno(errp, errno,
> -                         "Failed to unlink socket %s", un.sun_path);
> +                         "Failed to unlink socket %s", path);
>          goto err;
>      }
> +
> +    memset(&un, 0, sizeof(un));
> +    un.sun_family = AF_UNIX;
> +    strncpy(un.sun_path, path, sizeof(un.sun_path));
> +
>      if (bind(sock, (struct sockaddr*) &un, sizeof(un)) < 0) {
>          error_setg_errno(errp, errno, "Failed to bind socket to %s", 
> un.sun_path);

...you can see it in this bit of the context: this should be passing
"path" to the %s format string, shouldn't it?

Spotted looking at coverity issues, though unfortunately coverity
just always reports the "buffer not nul terminated" error rather
than only the cases where we don't nul terminate and then hand
the buffer to a function which consumes a nul-terminated string,
so we just have to mark the issue 'ignore' and don't get the
benefit of static checking :-(

thanks
-- PMM