Re: [PATCH] qemu-sockets: fix unix socket path copy (again)

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 31 Aug 2021 at 19:34, Michael Tokarev <mjt@tls.msk.ru> wrote:
>
> We test whenever the path of unix-domain socket
> address is non-empty and strictly-less than
> the length of the path buffer. Both these
> conditions are wrong: the socket can be unnamed,
> with empty path, or socket can have pathname
> null-terminated _after_ the sun_path buffer,
> since we provided more room when asking kernel.
>
> So on one side, allow empty, unnamed sockets
> (and adjust the test for abstract socket too -
> only do that if the socket is not unnamed),
> and on another side, allow path length to be
> up to our own maximum, - we have up to size
> of sockaddr_storage there.
>
> While at it, fix the duplication of regular
> pathname socket to not require trailing \0
> (since it can be missing for unnamed sockets).
>
> Fixes: 4cfd970ec188558daa6214f26203fe553fb1e01f (first in 6.1.0)
> Fixes: http://bugs.debian.org/993145
> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
> Cc: qemu-stable@nongnu.org
> --
> Two questions.
> 1. Why do we store the name of the socket to start with?
> 2. The code in the abstract socket case should not use
>    g_strndup but g_memdup instead, since the whole thing
>    is a blob of the given length, not a \0-terminated string.
> ---
>  util/qemu-sockets.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> index f2f3676d1f..7c83d81792 100644
> --- a/util/qemu-sockets.c
> +++ b/util/qemu-sockets.c
> @@ -1345,13 +1345,20 @@ socket_sockaddr_to_address_unix(struct sockaddr_storage *sa,
>      SocketAddress *addr;
>      struct sockaddr_un *su = (struct sockaddr_un *)sa;
>
> -    assert(salen >= sizeof(su->sun_family) + 1 &&
> -           salen <= sizeof(struct sockaddr_un));
> +    /* there's a corner case when trailing \0 does not fit into
> +     * sockaddr_un. Compare length with sizeof(sockaddr_storage),
> +     * not with sizeof(sockaddr_un), since this is what we actually
> +     * provide, to ensure we had no truncation and a room for
> +     * the trailing \0 which we add below.
> +     * When salen == sizeof(sun_family) it is unnamed socket,
> +     * and when first byte of sun_path is \0, it is abstract. */
> +    assert(salen >= sizeof(su->sun_family) &&
> +           salen <= sizeof(struct sockaddr_storage));

Again, why are we asserting an upper bound? We don't care here:
the representation in the SocketAddress structure has no length
limit on the path. (Conversely, we do care about the max length
when we convert from a SocketAddress to a sockaddr_un: we do this
in eg unix_connect_saddr().)

>      addr = g_new0(SocketAddress, 1);
>      addr->type = SOCKET_ADDRESS_TYPE_UNIX;
>  #ifdef CONFIG_LINUX
> -    if (!su->sun_path[0]) {
> +    if (salen > sizeof(su->sun_family) && !su->sun_path[0]) {
>          /* Linux abstract socket */
>          addr->u.q_unix.path = g_strndup(su->sun_path + 1,
>                                          salen - sizeof(su->sun_family) - 1);
> @@ -1363,7 +1370,7 @@ socket_sockaddr_to_address_unix(struct sockaddr_storage *sa,
>      }
>  #endif
>
> -    addr->u.q_unix.path = g_strndup(su->sun_path, sizeof(su->sun_path));
> +    addr->u.q_unix.path = g_strndup(su->sun_path, salen - sizeof(su->sun_family));
>      return addr;
>  }

-- PMM