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Date:   Thu, 15 Sep 2022 14:52:29 -0700
From:   Vinicius Costa Gomes <vinicius.gomes@...el.com>
To:     Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Voon Weifeng <weifeng.voon@...el.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net 1/2] net/sched: taprio: avoid disabling offload
 when it was never enabled

Vladimir Oltean <vladimir.oltean@....com> writes:

> In an incredibly strange API design decision, qdisc->destroy() gets
> called even if qdisc->init() never succeeded, not exclusively since
> commit 87b60cfacf9f ("net_sched: fix error recovery at qdisc creation"),
> but apparently also earlier (in the case of qdisc_create_dflt()).
>
> The taprio qdisc does not fully acknowledge this when it attempts full
> offload, because it starts off with q->flags = TAPRIO_FLAGS_INVALID in
> taprio_init(), then it replaces q->flags with TCA_TAPRIO_ATTR_FLAGS
> parsed from netlink (in taprio_change(), tail called from taprio_init()).
>
> But in taprio_destroy(), we call taprio_disable_offload(), and this
> determines what to do based on FULL_OFFLOAD_IS_ENABLED(q->flags).
>
> But looking at the implementation of FULL_OFFLOAD_IS_ENABLED()
> (a bitwise check of bit 1 in q->flags), it is invalid to call this macro
> on q->flags when it contains TAPRIO_FLAGS_INVALID, because that is set
> to U32_MAX, and therefore FULL_OFFLOAD_IS_ENABLED() will return true on
> an invalid set of flags.
>
> As a result, it is possible to crash the kernel if user space forces an
> error between setting q->flags = TAPRIO_FLAGS_INVALID, and the calling
> of taprio_enable_offload(). This is because drivers do not expect the
> offload to be disabled when it was never enabled.
>
> The error that we force here is to attach taprio as a non-root qdisc,
> but instead as child of an mqprio root qdisc:
>
> $ tc qdisc add dev swp0 root handle 1: \
> 	mqprio num_tc 8 map 0 1 2 3 4 5 6 7 \
> 	queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 hw 0
> $ tc qdisc replace dev swp0 parent 1:1 \
> 	taprio num_tc 8 map 0 1 2 3 4 5 6 7 \
> 	queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 0 \
> 	sched-entry S 0x7f 990000 sched-entry S 0x80 100000 \
> 	flags 0x0 clockid CLOCK_TAI
> Unable to handle kernel paging request at virtual address fffffffffffffff8
> [fffffffffffffff8] pgd=0000000000000000, p4d=0000000000000000
> Internal error: Oops: 96000004 [#1] PREEMPT SMP
> Call trace:
>  taprio_dump+0x27c/0x310
>  vsc9959_port_setup_tc+0x1f4/0x460
>  felix_port_setup_tc+0x24/0x3c
>  dsa_slave_setup_tc+0x54/0x27c
>  taprio_disable_offload.isra.0+0x58/0xe0
>  taprio_destroy+0x80/0x104
>  qdisc_create+0x240/0x470
>  tc_modify_qdisc+0x1fc/0x6b0
>  rtnetlink_rcv_msg+0x12c/0x390
>  netlink_rcv_skb+0x5c/0x130
>  rtnetlink_rcv+0x1c/0x2c
>
> Fix this by keeping track of the operations we made, and undo the
> offload only if we actually did it.
>
> I've added "bool offloaded" inside a 4 byte hole between "int clockid"
> and "atomic64_t picos_per_byte". Now the first cache line looks like
> below:
>
> $ pahole -C taprio_sched net/sched/sch_taprio.o
> struct taprio_sched {
>         struct Qdisc * *           qdiscs;               /*     0     8 */
>         struct Qdisc *             root;                 /*     8     8 */
>         u32                        flags;                /*    16     4 */
>         enum tk_offsets            tk_offset;            /*    20     4 */
>         int                        clockid;              /*    24     4 */
>         bool                       offloaded;            /*    28     1 */
>
>         /* XXX 3 bytes hole, try to pack */
>
>         atomic64_t                 picos_per_byte;       /*    32     0 */
>
>         /* XXX 8 bytes hole, try to pack */
>
>         spinlock_t                 current_entry_lock;   /*    40     0 */
>
>         /* XXX 8 bytes hole, try to pack */
>
>         struct sched_entry *       current_entry;        /*    48     8 */
>         struct sched_gate_list *   oper_sched;           /*    56     8 */
>         /* --- cacheline 1 boundary (64 bytes) --- */
>
> Fixes: 9c66d1564676 ("taprio: Add support for hardware offloading")
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> ---

Reviewed-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>


Cheers,
-- 
Vinicius

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