netdev - Re: [PATCH bpf] libbpf: fix using uninitialized ioctl results

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Message-ID: <CAADnVQ+EJ6WArycRgVePTRrqerpt9E47H5EPm7APX5Jjmv-GWw@mail.gmail.com>
Date:   Tue, 23 Jul 2019 15:18:49 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Ilya Maximets <i.maximets@...sung.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf] libbpf: fix using uninitialized ioctl results

On Tue, Jul 23, 2019 at 5:08 AM Ilya Maximets <i.maximets@...sung.com> wrote:
>
> 'channels.max_combined' initialized only on ioctl success and
> errno is only valid on ioctl failure.
>
> The code doesn't produce any runtime issues, but makes memory
> sanitizers angry:
>
>  Conditional jump or move depends on uninitialised value(s)
>     at 0x55C056F: xsk_get_max_queues (xsk.c:336)
>     by 0x55C05B2: xsk_create_bpf_maps (xsk.c:354)
>     by 0x55C089F: xsk_setup_xdp_prog (xsk.c:447)
>     by 0x55C0E57: xsk_socket__create (xsk.c:601)
>   Uninitialised value was created by a stack allocation
>     at 0x55C04CD: xsk_get_max_queues (xsk.c:318)
>
> Additionally fixed warning on uninitialized bytes in ioctl arguments:
>
>  Syscall param ioctl(SIOCETHTOOL) points to uninitialised byte(s)
>     at 0x648D45B: ioctl (in /usr/lib64/libc-2.28.so)
>     by 0x55C0546: xsk_get_max_queues (xsk.c:330)
>     by 0x55C05B2: xsk_create_bpf_maps (xsk.c:354)
>     by 0x55C089F: xsk_setup_xdp_prog (xsk.c:447)
>     by 0x55C0E57: xsk_socket__create (xsk.c:601)
>   Address 0x1ffefff378 is on thread 1's stack
>   in frame #1, created by xsk_get_max_queues (xsk.c:318)
>   Uninitialised value was created by a stack allocation
>     at 0x55C04CD: xsk_get_max_queues (xsk.c:318)
>
> CC: Magnus Karlsson <magnus.karlsson@...el.com>
> Fixes: 1cad07884239 ("libbpf: add support for using AF_XDP sockets")
> Signed-off-by: Ilya Maximets <i.maximets@...sung.com>
> ---
>  tools/lib/bpf/xsk.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index 5007b5d4fd2c..c4f912dc30f9 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -317,7 +317,7 @@ static int xsk_load_xdp_prog(struct xsk_socket *xsk)
>
>  static int xsk_get_max_queues(struct xsk_socket *xsk)
>  {
> -       struct ethtool_channels channels;
> +       struct ethtool_channels channels = { .cmd = ETHTOOL_GCHANNELS };
>         struct ifreq ifr;
>         int fd, err, ret;
>
> @@ -325,7 +325,7 @@ static int xsk_get_max_queues(struct xsk_socket *xsk)
>         if (fd < 0)
>                 return -errno;
>
> -       channels.cmd = ETHTOOL_GCHANNELS;
> +       memset(&ifr, 0, sizeof(ifr));

I refactored this bit into
struct ifreq ifr = {};
based on Andrii suggestion and pushed to bpf tree.
Thanks

>         ifr.ifr_data = (void *)&channels;
>         strncpy(ifr.ifr_name, xsk->ifname, IFNAMSIZ - 1);
>         ifr.ifr_name[IFNAMSIZ - 1] = '\0';
> @@ -335,7 +335,7 @@ static int xsk_get_max_queues(struct xsk_socket *xsk)
>                 goto out;
>         }
>
> -       if (channels.max_combined == 0 || errno == EOPNOTSUPP)
> +       if (err || channels.max_combined == 0)
>                 /* If the device says it has no channels, then all traffic
>                  * is sent to a single stream, so max queues = 1.
>                  */
> --
> 2.17.1
>