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    Subject[PATCH 5.4 257/260] fq_codel: reject silly quantum parameters
    Date
    From: Eric Dumazet <edumazet@google.com>

    [ Upstream commit c7c5e6ff533fe1f9afef7d2fa46678987a1335a7 ]

    syzbot found that forcing a big quantum attribute would crash hosts fast,
    essentially using this:

    tc qd replace dev eth0 root fq_codel quantum 4294967295

    This is because fq_codel_dequeue() would have to loop
    ~2^31 times in :

    if (flow->deficit <= 0) {
    flow->deficit += q->quantum;
    list_move_tail(&flow->flowchain, &q->old_flows);
    goto begin;
    }

    SFQ max quantum is 2^19 (half a megabyte)
    Lets adopt a max quantum of one megabyte for FQ_CODEL.

    Fixes: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reported-by: syzbot <syzkaller@googlegroups.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    include/uapi/linux/pkt_sched.h | 2 ++
    net/sched/sch_fq_codel.c | 12 ++++++++++--
    2 files changed, 12 insertions(+), 2 deletions(-)

    diff --git a/include/uapi/linux/pkt_sched.h b/include/uapi/linux/pkt_sched.h
    index edbbf4bfdd9e..4a245d7a5c8d 100644
    --- a/include/uapi/linux/pkt_sched.h
    +++ b/include/uapi/linux/pkt_sched.h
    @@ -807,6 +807,8 @@ struct tc_codel_xstats {

    /* FQ_CODEL */

    +#define FQ_CODEL_QUANTUM_MAX (1 << 20)
    +
    enum {
    TCA_FQ_CODEL_UNSPEC,
    TCA_FQ_CODEL_TARGET,
    diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
    index 76d72c3f52ed..86fb2f953bd5 100644
    --- a/net/sched/sch_fq_codel.c
    +++ b/net/sched/sch_fq_codel.c
    @@ -370,6 +370,7 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
    {
    struct fq_codel_sched_data *q = qdisc_priv(sch);
    struct nlattr *tb[TCA_FQ_CODEL_MAX + 1];
    + u32 quantum = 0;
    int err;

    if (!opt)
    @@ -387,6 +388,13 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
    q->flows_cnt > 65536)
    return -EINVAL;
    }
    + if (tb[TCA_FQ_CODEL_QUANTUM]) {
    + quantum = max(256U, nla_get_u32(tb[TCA_FQ_CODEL_QUANTUM]));
    + if (quantum > FQ_CODEL_QUANTUM_MAX) {
    + NL_SET_ERR_MSG(extack, "Invalid quantum");
    + return -EINVAL;
    + }
    + }
    sch_tree_lock(sch);

    if (tb[TCA_FQ_CODEL_TARGET]) {
    @@ -413,8 +421,8 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
    if (tb[TCA_FQ_CODEL_ECN])
    q->cparams.ecn = !!nla_get_u32(tb[TCA_FQ_CODEL_ECN]);

    - if (tb[TCA_FQ_CODEL_QUANTUM])
    - q->quantum = max(256U, nla_get_u32(tb[TCA_FQ_CODEL_QUANTUM]));
    + if (quantum)
    + q->quantum = quantum;

    if (tb[TCA_FQ_CODEL_DROP_BATCH_SIZE])
    q->drop_batch_size = max(1U, nla_get_u32(tb[TCA_FQ_CODEL_DROP_BATCH_SIZE]));
    --
    2.30.2


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