Re: [PATCH net 2/2] net/sched: qdisc_destroy() old ingress and clsact Qdiscs before grafting

[Paolo Abeni]

Hi,

On Sat, 2023-06-10 at 20:30 -0700, Peilin Ye wrote:
> From: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>
> 
> 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 {ingress,clsact}_destroy()) has
> finished:
> 
> In qdisc_graft(), return -EBUSY if @old has any ongoing 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 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 commit c7cfbd115001 ("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 just one of the possible consequences of concurrently accessing
> miniq_{in,e}gress pointers.
> 
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/r/0000000000006cf87705f79acf1a@xxxxxxxxxx/
> Cc: Hillf Danton <hdanton@xxxxxxxx>
> Cc: Vlad Buslov <vladbu@xxxxxxxxxxxx>
> Signed-off-by: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>

The fixes LGTM, but I guess this could deserve an explicit ack from
Jakub, as he raised to point for the full retry implementation.

Cheers,

Paolo