Re: [PATCH v5 net 6/6] net/sched: qdisc_destroy() old ingress and clsact Qdiscs before grafting

Jamal Hadi Salim
2023-05-26T12:21:08Z

On Wed, May 24, 2023 at 11:39 AM Pedro Tammela <pctammela@mojatatu.com> wrote:
>
> On 23/05/2023 22:20, Peilin Ye wrote:
> > From: Peilin Ye <peilin.ye@bytedance.com>
> >
> > mini_Qdisc_pair::p_miniq is a double pointer to mini_Qdisc, initialized in
> > ingress_init() to point to net_device::miniq_ingress. ingress Qdiscs
> > access this per-net_device pointer in mini_qdisc_pair_swap(). Similar for
> > clsact Qdiscs and miniq_egress.
> >
> > Unfortunately, after introducing RTNL-unlocked RTM_{NEW,DEL,GET}TFILTER
> > requests (thanks Hillf Danton for the hint), when replacing ingress or
> > clsact Qdiscs, for example, the old Qdisc ("@old") could access the same
> > miniq_{in,e}gress pointer(s) concurrently with the new Qdisc ("@new"),
> > causing race conditions [1] including a use-after-free bug in
> > mini_qdisc_pair_swap() reported by syzbot:
> >
> > BUG: KASAN: slab-use-after-free in mini_qdisc_pair_swap+0x1c2/0x1f0 net/sched/sch_generic.c:1573
> > Write of size 8 at addr ffff888045b31308 by task syz-executor690/14901
> > ...
> > Call Trace:
> > <TASK>
> > __dump_stack lib/dump_stack.c:88 [inline]
> > dump_stack_lvl+0xd9/0x150 lib/dump_stack.c:106
> > print_address_description.constprop.0+0x2c/0x3c0 mm/kasan/report.c:319
> > print_report mm/kasan/report.c:430 [inline]
> > kasan_report+0x11c/0x130 mm/kasan/report.c:536
> > mini_qdisc_pair_swap+0x1c2/0x1f0 net/sched/sch_generic.c:1573
> > tcf_chain_head_change_item net/sched/cls_api.c:495 [inline]
> > tcf_chain0_head_change.isra.0+0xb9/0x120 net/sched/cls_api.c:509
> > tcf_chain_tp_insert net/sched/cls_api.c:1826 [inline]
> > tcf_chain_tp_insert_unique net/sched/cls_api.c:1875 [inline]
> > tc_new_tfilter+0x1de6/0x2290 net/sched/cls_api.c:2266
> > ...
> >
> > @old and @new should not affect each other. In other words, @old should
> > never modify miniq_{in,e}gress after @new, and @new should not update
> > @old's RCU state. Fixing without changing sch_api.c turned out to be
> > difficult (please refer to Closes: for discussions). Instead, make sure
> > @new's first call always happen after @old's last call, in
> > qdisc_destroy(), has finished:
> >
> > In qdisc_graft(), return -EAGAIN and tell the caller to replay
> > (suggested by Vlad Buslov) if @old has any ongoing RTNL-unlocked filter
> > requests, and call qdisc_destroy() for @old before grafting @new.
> >
> > Introduce qdisc_refcount_dec_if_one() as the counterpart of
> > qdisc_refcount_inc_nz() used for RTNL-unlocked filter requests. Introduce
> > a non-static version of qdisc_destroy() that does a TCQ_F_BUILTIN check,
> > just like qdisc_put() etc.
> >
> > Depends on patch "net/sched: Refactor qdisc_graft() for ingress and clsact
> > Qdiscs".
> >
> > [1] To illustrate, the syzkaller reproducer adds ingress Qdiscs under
> > TC_H_ROOT (no longer possible after patch "net/sched: sch_ingress: Only
> > create under TC_H_INGRESS") on eth0 that has 8 transmission queues:
> >
> > Thread 1 creates ingress Qdisc A (containing mini Qdisc a1 and a2), then
> > adds a flower filter X to A.
> >
> > Thread 2 creates another ingress Qdisc B (containing mini Qdisc b1 and
> > b2) to replace A, then adds a flower filter Y to B.
> >
> > Thread 1 A's refcnt Thread 2
> > RTM_NEWQDISC (A, RTNL-locked)
> > qdisc_create(A) 1
> > qdisc_graft(A) 9
> >
> > RTM_NEWTFILTER (X, RTNL-unlocked)
> > __tcf_qdisc_find(A) 10
> > tcf_chain0_head_change(A)
> > mini_qdisc_pair_swap(A) (1st)
> > |
> > | RTM_NEWQDISC (B, RTNL-locked)
> > RCU sync 2 qdisc_graft(B)
> > | 1 notify_and_destroy(A)
> > |
> > tcf_block_release(A) 0 RTM_NEWTFILTER (Y, RTNL-unlocked)
> > qdisc_destroy(A) tcf_chain0_head_change(B)
> > tcf_chain0_head_change_cb_del(A) mini_qdisc_pair_swap(B) (2nd)
> > mini_qdisc_pair_swap(A) (3rd) |
> > ... ...
> >
> > Here, B calls mini_qdisc_pair_swap(), pointing eth0->miniq_ingress to its
> > mini Qdisc, b1. Then, A calls mini_qdisc_pair_swap() again during
> > ingress_destroy(), setting eth0->miniq_ingress to NULL, so ingress packets
> > on eth0 will not find filter Y in sch_handle_ingress().
> >
> > This is only one of the possible consequences of concurrently accessing
> > miniq_{in,e}gress pointers. The point is clear though: again, A should
> > never modify those per-net_device pointers after B, and B should not
> > update A's RCU state.
> >
> > Fixes: 7a096d579e8e ("net: sched: ingress: set 'unlocked' flag for Qdisc ops")
> > Fixes: 87f373921c4e ("net: sched: ingress: set 'unlocked' flag for clsact Qdisc ops")
> > Reported-by: syzbot+b53a9c0d1ea4ad62da8b@syzkaller.appspotmail.com
> > Closes: https://lore.kernel.org/r/0000000000006cf87705f79acf1a@google.com/
> > Cc: Hillf Danton <hdanton@sina.com>
> > Cc: Vlad Buslov <vladbu@mellanox.com>
> > Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
>
> Tested-by: Pedro Tammela <pctammela@mojatatu.com>

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>

cheers,
jamal