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    Subject[PATCH 5.4 60/77] bpf: Dont redirect packets with invalid pkt_len
    Date
    From: Zhengchao Shao <shaozhengchao@huawei.com>

    commit fd1894224407c484f652ad456e1ce423e89bb3eb upstream.

    Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout any
    skbs, that is, the flow->head is null.
    The root cause, as the [2] says, is because that bpf_prog_test_run_skb()
    run a bpf prog which redirects empty skbs.
    So we should determine whether the length of the packet modified by bpf
    prog or others like bpf_prog_test is valid before forwarding it directly.

    LINK: [1] https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16ec96c5
    LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html

    Reported-by: syzbot+7a12909485b94426aceb@syzkaller.appspotmail.com
    Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
    Reviewed-by: Stanislav Fomichev <sdf@google.com>
    Link: https://lore.kernel.org/r/20220715115559.139691-1-shaozhengchao@huawei.com
    Signed-off-by: Alexei Starovoitov <ast@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    include/linux/skbuff.h | 8 ++++++++
    net/bpf/test_run.c | 3 +++
    net/core/dev.c | 1 +
    3 files changed, 12 insertions(+)

    --- a/include/linux/skbuff.h
    +++ b/include/linux/skbuff.h
    @@ -2201,6 +2201,14 @@ static inline void skb_set_tail_pointer(

    #endif /* NET_SKBUFF_DATA_USES_OFFSET */

    +static inline void skb_assert_len(struct sk_buff *skb)
    +{
    +#ifdef CONFIG_DEBUG_NET
    + if (WARN_ONCE(!skb->len, "%s\n", __func__))
    + DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
    +#endif /* CONFIG_DEBUG_NET */
    +}
    +
    /*
    * Add data to an sk_buff
    */
    --- a/net/bpf/test_run.c
    +++ b/net/bpf/test_run.c
    @@ -200,6 +200,9 @@ static int convert___skb_to_skb(struct s
    {
    struct qdisc_skb_cb *cb = (struct qdisc_skb_cb *)skb->cb;

    + if (!skb->len)
    + return -EINVAL;
    +
    if (!__skb)
    return 0;

    --- a/net/core/dev.c
    +++ b/net/core/dev.c
    @@ -3712,6 +3712,7 @@ static int __dev_queue_xmit(struct sk_bu
    bool again = false;

    skb_reset_mac_header(skb);
    + skb_assert_len(skb);

    if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_SCHED_TSTAMP))
    __skb_tstamp_tx(skb, NULL, skb->sk, SCM_TSTAMP_SCHED);

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