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    Subject[PATCH 4.19 625/639] packet: fix data-race in fanout_flow_is_huge()
    Date
    From: Eric Dumazet <edumazet@google.com>

    [ Upstream commit b756ad928d98e5ef0b74af7546a6a31a8dadde00 ]

    KCSAN reported the following data-race [1]

    Adding a couple of READ_ONCE()/WRITE_ONCE() should silence it.

    Since the report hinted about multiple cpus using the history
    concurrently, I added a test avoiding writing on it if the
    victim slot already contains the desired value.

    [1]

    BUG: KCSAN: data-race in fanout_demux_rollover / fanout_demux_rollover

    read to 0xffff8880b01786cc of 4 bytes by task 18921 on cpu 1:
    fanout_flow_is_huge net/packet/af_packet.c:1303 [inline]
    fanout_demux_rollover+0x33e/0x3f0 net/packet/af_packet.c:1353
    packet_rcv_fanout+0x34e/0x490 net/packet/af_packet.c:1453
    deliver_skb net/core/dev.c:1888 [inline]
    dev_queue_xmit_nit+0x15b/0x540 net/core/dev.c:1958
    xmit_one net/core/dev.c:3195 [inline]
    dev_hard_start_xmit+0x3f5/0x430 net/core/dev.c:3215
    __dev_queue_xmit+0x14ab/0x1b40 net/core/dev.c:3792
    dev_queue_xmit+0x21/0x30 net/core/dev.c:3825
    neigh_direct_output+0x1f/0x30 net/core/neighbour.c:1530
    neigh_output include/net/neighbour.h:511 [inline]
    ip6_finish_output2+0x7a2/0xec0 net/ipv6/ip6_output.c:116
    __ip6_finish_output net/ipv6/ip6_output.c:142 [inline]
    __ip6_finish_output+0x2d7/0x330 net/ipv6/ip6_output.c:127
    ip6_finish_output+0x41/0x160 net/ipv6/ip6_output.c:152
    NF_HOOK_COND include/linux/netfilter.h:294 [inline]
    ip6_output+0xf2/0x280 net/ipv6/ip6_output.c:175
    dst_output include/net/dst.h:436 [inline]
    ip6_local_out+0x74/0x90 net/ipv6/output_core.c:179
    ip6_send_skb+0x53/0x110 net/ipv6/ip6_output.c:1795
    udp_v6_send_skb.isra.0+0x3ec/0xa70 net/ipv6/udp.c:1173
    udpv6_sendmsg+0x1906/0x1c20 net/ipv6/udp.c:1471
    inet6_sendmsg+0x6d/0x90 net/ipv6/af_inet6.c:576
    sock_sendmsg_nosec net/socket.c:637 [inline]
    sock_sendmsg+0x9f/0xc0 net/socket.c:657
    ___sys_sendmsg+0x2b7/0x5d0 net/socket.c:2311
    __sys_sendmmsg+0x123/0x350 net/socket.c:2413
    __do_sys_sendmmsg net/socket.c:2442 [inline]
    __se_sys_sendmmsg net/socket.c:2439 [inline]
    __x64_sys_sendmmsg+0x64/0x80 net/socket.c:2439
    do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

    write to 0xffff8880b01786cc of 4 bytes by task 18922 on cpu 0:
    fanout_flow_is_huge net/packet/af_packet.c:1306 [inline]
    fanout_demux_rollover+0x3a4/0x3f0 net/packet/af_packet.c:1353
    packet_rcv_fanout+0x34e/0x490 net/packet/af_packet.c:1453
    deliver_skb net/core/dev.c:1888 [inline]
    dev_queue_xmit_nit+0x15b/0x540 net/core/dev.c:1958
    xmit_one net/core/dev.c:3195 [inline]
    dev_hard_start_xmit+0x3f5/0x430 net/core/dev.c:3215
    __dev_queue_xmit+0x14ab/0x1b40 net/core/dev.c:3792
    dev_queue_xmit+0x21/0x30 net/core/dev.c:3825
    neigh_direct_output+0x1f/0x30 net/core/neighbour.c:1530
    neigh_output include/net/neighbour.h:511 [inline]
    ip6_finish_output2+0x7a2/0xec0 net/ipv6/ip6_output.c:116
    __ip6_finish_output net/ipv6/ip6_output.c:142 [inline]
    __ip6_finish_output+0x2d7/0x330 net/ipv6/ip6_output.c:127
    ip6_finish_output+0x41/0x160 net/ipv6/ip6_output.c:152
    NF_HOOK_COND include/linux/netfilter.h:294 [inline]
    ip6_output+0xf2/0x280 net/ipv6/ip6_output.c:175
    dst_output include/net/dst.h:436 [inline]
    ip6_local_out+0x74/0x90 net/ipv6/output_core.c:179
    ip6_send_skb+0x53/0x110 net/ipv6/ip6_output.c:1795
    udp_v6_send_skb.isra.0+0x3ec/0xa70 net/ipv6/udp.c:1173
    udpv6_sendmsg+0x1906/0x1c20 net/ipv6/udp.c:1471
    inet6_sendmsg+0x6d/0x90 net/ipv6/af_inet6.c:576
    sock_sendmsg_nosec net/socket.c:637 [inline]
    sock_sendmsg+0x9f/0xc0 net/socket.c:657
    ___sys_sendmsg+0x2b7/0x5d0 net/socket.c:2311
    __sys_sendmmsg+0x123/0x350 net/socket.c:2413
    __do_sys_sendmmsg net/socket.c:2442 [inline]
    __se_sys_sendmmsg net/socket.c:2439 [inline]
    __x64_sys_sendmmsg+0x64/0x80 net/socket.c:2439
    do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

    Reported by Kernel Concurrency Sanitizer on:
    CPU: 0 PID: 18922 Comm: syz-executor.3 Not tainted 5.4.0-rc6+ #0
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

    Fixes: 3b3a5b0aab5b ("packet: rollover huge flows before small flows")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Cc: Willem de Bruijn <willemb@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    net/packet/af_packet.c | 12 +++++++++---
    1 file changed, 9 insertions(+), 3 deletions(-)

    diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
    index 60ba18a4bb0ff..ddf90e6fac514 100644
    --- a/net/packet/af_packet.c
    +++ b/net/packet/af_packet.c
    @@ -1297,15 +1297,21 @@ static void packet_sock_destruct(struct sock *sk)

    static bool fanout_flow_is_huge(struct packet_sock *po, struct sk_buff *skb)
    {
    - u32 rxhash;
    + u32 *history = po->rollover->history;
    + u32 victim, rxhash;
    int i, count = 0;

    rxhash = skb_get_hash(skb);
    for (i = 0; i < ROLLOVER_HLEN; i++)
    - if (po->rollover->history[i] == rxhash)
    + if (READ_ONCE(history[i]) == rxhash)
    count++;

    - po->rollover->history[prandom_u32() % ROLLOVER_HLEN] = rxhash;
    + victim = prandom_u32() % ROLLOVER_HLEN;
    +
    + /* Avoid dirtying the cache line if possible */
    + if (READ_ONCE(history[victim]) != rxhash)
    + WRITE_ONCE(history[victim], rxhash);
    +
    return count > (ROLLOVER_HLEN >> 1);
    }

    --
    2.20.1


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