Re: [Qemu-devel] [PATCH v3] sockets: improve error reporting if UNIX socket path is too long

[Eric Blake]

On 05/25/2017 05:19 AM, Daniel P. Berrange wrote:
> 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>
> ---
> 

>      } else {
>          const char *tmpdir = getenv("TMPDIR");
>          tmpdir = tmpdir ? tmpdir : "/tmp";
> -        if (snprintf(un.sun_path, sizeof(un.sun_path), 
> "%s/qemu-socket-XXXXXX",
> -                     tmpdir) >= sizeof(un.sun_path)) {
> -            error_setg_errno(errp, errno,
> -                             "TMPDIR environment variable (%s) too large", 
> tmpdir);
> -            goto err;

The old code fails early if the generated name is too long,...

> -        }
> +        path = pathbuf = g_strdup_printf("%s/qemu-socket-XXXXXX", tmpdir);
>  
>          /*
>           * This dummy fd usage silences the mktemp() unsecure warning.
> @@ -873,24 +868,32 @@ 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);

...while the new code ends up creating a too-long name in the
file-system anyways,...

>          if (fd < 0) {
>              error_setg_errno(errp, errno,
>                               "Failed to make a temporary socket name in %s", 
> tmpdir);
>              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 (strlen(path) > sizeof(un.sun_path)) {
> +        error_setg(errp, "UNIX socket path '%s' is too long", path);
> +        error_append_hint(errp, "Path must be less than %zu bytes\n",
> +                          sizeof(un.sun_path));

The old message mentioned the name that was too long, the new does not.
That's a potential slight loss in error message quality, particularly
since you are no longer mentioning a UNIX socket (which may give the
user a clue why 108 is special), but I can probably live with it (it's
still pretty obvious from the message that you should investigate what
is too long).

> +        goto err;

...here, you finally diagnose that the name is too long,...

> +    }
> +
> +    if (unlink(path) < 0 && errno != ENOENT) {

...but skip the unlink of the just-created too-long name.  Oops.

>          error_setg_errno(errp, errno,
> -                         "Failed to unlink socket %s", un.sun_path);
> +                         "Failed to unlink socket %s", path);
>          goto err;
>      }

I think you're okay if you just swap the unlink() with the length check.
 Although it seems rather sys-call heavy to mkstemp/unlink compared to
just doing a length check first, I do like how your refactoring ended up
with fewer conditionals rather than splitting the generated name code
across multiple conditionals.

> @@ -932,9 +942,16 @@ static int unix_connect_saddr(UnixSocketAddress *saddr,
>          qemu_set_nonblock(sock);
>      }
>  
> +    if (strlen(saddr->path) > sizeof(un.sun_path)) {
> +        error_setg(errp, "UNIX socket path '%s' is too long", saddr->path);
> +        error_append_hint(errp, "Path must be less than %zu bytes\n",
> +                          sizeof(un.sun_path));

Again, potential loss in error message quality.

Hopefully fourth time is the charm!

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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