netdev - Re: [PATCH net] net-timestamp: convert sk->sk_tskey to atomic

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Date:   Thu, 17 Feb 2022 18:18:34 +0100
From:   Marco Elver <elver@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] net-timestamp: convert sk->sk_tskey to atomic_t

On Thu, 17 Feb 2022 at 18:05, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> UDP sendmsg() can be lockless, this is causing all kinds
> of data races.
>
> This patch converts sk->sk_tskey to remove one of these races.
>
> BUG: KCSAN: data-race in __ip_append_data / __ip_append_data
>
> read to 0xffff8881035d4b6c of 4 bytes by task 8877 on cpu 1:
>  __ip_append_data+0x1c1/0x1de0 net/ipv4/ip_output.c:994
>  ip_make_skb+0x13f/0x2d0 net/ipv4/ip_output.c:1636
>  udp_sendmsg+0x12bd/0x14c0 net/ipv4/udp.c:1249
>  inet_sendmsg+0x5f/0x80 net/ipv4/af_inet.c:819
>  sock_sendmsg_nosec net/socket.c:705 [inline]
>  sock_sendmsg net/socket.c:725 [inline]
>  ____sys_sendmsg+0x39a/0x510 net/socket.c:2413
>  ___sys_sendmsg net/socket.c:2467 [inline]
>  __sys_sendmmsg+0x267/0x4c0 net/socket.c:2553
>  __do_sys_sendmmsg net/socket.c:2582 [inline]
>  __se_sys_sendmmsg net/socket.c:2579 [inline]
>  __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2579
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> write to 0xffff8881035d4b6c of 4 bytes by task 8880 on cpu 0:
>  __ip_append_data+0x1d8/0x1de0 net/ipv4/ip_output.c:994
>  ip_make_skb+0x13f/0x2d0 net/ipv4/ip_output.c:1636
>  udp_sendmsg+0x12bd/0x14c0 net/ipv4/udp.c:1249
>  inet_sendmsg+0x5f/0x80 net/ipv4/af_inet.c:819
>  sock_sendmsg_nosec net/socket.c:705 [inline]
>  sock_sendmsg net/socket.c:725 [inline]
>  ____sys_sendmsg+0x39a/0x510 net/socket.c:2413
>  ___sys_sendmsg net/socket.c:2467 [inline]
>  __sys_sendmmsg+0x267/0x4c0 net/socket.c:2553
>  __do_sys_sendmmsg net/socket.c:2582 [inline]
>  __se_sys_sendmmsg net/socket.c:2579 [inline]
>  __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2579
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> value changed: 0x0000054d -> 0x0000054e
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 8880 Comm: syz-executor.5 Not tainted 5.17.0-rc2-syzkaller-00167-gdcb85f85fa6f-dirty #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>
> Fixes: 09c2d251b707 ("net-timestamp: add key to disambiguate concurrent datagrams")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>
> ---
>  include/net/sock.h        | 4 ++--
>  net/can/j1939/transport.c | 2 +-
>  net/core/skbuff.c         | 2 +-
>  net/core/sock.c           | 4 ++--
>  net/ipv4/ip_output.c      | 2 +-
>  net/ipv6/ip6_output.c     | 2 +-
>  6 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index ff9b508d9c5ffcb9a30deb730b27046e463bda37..50aecd28b355082bce495a89a8a871b15e3e7e2c 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -507,7 +507,7 @@ struct sock {
>  #endif
>         u16                     sk_tsflags;
>         u8                      sk_shutdown;
> -       u32                     sk_tskey;
> +       atomic_t                sk_tskey;
>         atomic_t                sk_zckey;
>
>         u8                      sk_clockid;
> @@ -2667,7 +2667,7 @@ static inline void _sock_tx_timestamp(struct sock *sk, __u16 tsflags,
>                 __sock_tx_timestamp(tsflags, tx_flags);
>                 if (tsflags & SOF_TIMESTAMPING_OPT_ID && tskey &&
>                     tsflags & SOF_TIMESTAMPING_TX_RECORD_MASK)
> -                       *tskey = sk->sk_tskey++;
> +                       *tskey = atomic_inc_return(&sk->sk_tskey) - 1;
>         }
>         if (unlikely(sock_flag(sk, SOCK_WIFI_STATUS)))
>                 *tx_flags |= SKBTX_WIFI_STATUS;
> diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
> index a271688780a2c1a3bff6c2578502f972da34a30b..307ee1174a6e2e3d8cb9edd2c7485ddd22014ce6 100644
> --- a/net/can/j1939/transport.c
> +++ b/net/can/j1939/transport.c
> @@ -2006,7 +2006,7 @@ struct j1939_session *j1939_tp_send(struct j1939_priv *priv,
>                 /* set the end-packet for broadcast */
>                 session->pkt.last = session->pkt.total;
>
> -       skcb->tskey = session->sk->sk_tskey++;
> +       skcb->tskey = atomic_inc_return(&session->sk->sk_tskey) - 1;

We also have atomic_fetch_inc() in case it'd make this simpler.

Thanks,
-- Marco