Re: [syzbot] [net?] KASAN: slab-use-after-free Write in mini_qdisc_pair_swap

[Vlad Buslov]

On Thu 27 Apr 2023 at 10:35, Peilin Ye <yepeilin.cs@xxxxxxxxx> wrote:
> Hi Pedro, Vlad,
>
> On Thu, Apr 27, 2023 at 03:26:03PM +0300, Vlad Buslov wrote:
>> On Wed 26 Apr 2023 at 16:42, Peilin Ye <yepeilin.cs@xxxxxxxxx> wrote:
>> > As we can see there're interleaving mini_qdisc_pair_swap() calls between
>> > Qdisc A and B, causing all kinds of troubles, including the UAF (thread
>> > 2 writing to mini Qdisc a1's rcu_state after Qdisc A has already been
>> > freed) reported by syzbot.
>> 
>> Great analysis! However, it is still not quite clear to me how threads 1
>> and 2 access each other RCU state when q->miniqp is a private memory of
>> the Qdisc, so 1 should only see A->miniqp and 2 only B->miniqp. And both
>> miniqps should be protected from deallocation by reference that lockless
>> RTM_NEWTFILTER obtains.
>
> Thanks for taking a look!
>
> To elaborate, p_miniq is a pointer of pointer of struct mini_Qdisc,
> initialized in ingress_init() to point to eth0->miniq_ingress, which
> isn't private to A or B.
>
> In other words, both A->miniqp->p_miniq and B->miniqp->p_miniq point to
> eth0->miniq_ingress.
>
> For your reference, roughly speaking, mini_qdisc_pair_swap() does this:
>
>   miniq_old = dev->miniq_ingress;
>
>   if (destroying) {
>           dev->miniq_ingress = NULL;
>   } else {
>           rcu_wait();
>           dev->miniq_ingress = miniq_new;
>   }
>
>   if (miniq_old)
>           miniq_old->rcu_state = ...
>
> On Wed 26 Apr 2023 at 16:42, Peilin Ye <yepeilin.cs@xxxxxxxxx> wrote:
>>  Thread 1               A's refcnt   Thread 2
>>   RTM_NEWQDISC (A, locked)
>>    qdisc_create(A)               1
>>    qdisc_graft(A)                9
>>
>>   RTM_NEWTFILTER (X, lockless)
>>    __tcf_qdisc_find(A)          10
>>    tcf_chain0_head_change(A)
>>  ! mini_qdisc_pair_swap(A)
>
>   1. A adds its first filter,
>      miniq_old (eth0->miniq_ingress) is NULL,
>      RCU wait starts,
>      RCU wait ends,
>      change eth0->miniq_ingress to A's mini Qdisc.
>
>>             |                        RTM_NEWQDISC (B, locked)
>>             |                    2    qdisc_graft(B)
>>             |                    1    notify_and_destroy(A)
>>             |
>>             |                        RTM_NEWTFILTER (Y, lockless)
>>             |                         tcf_chain0_head_change(B)
>>             |                       ! mini_qdisc_pair_swap(B)
>
>                       2. B adds its first filter,
>                          miniq_old (eth0->miniq_ingress) is A's mini Qdisc,
>                          RCU wait starts,
>
>>    tcf_block_release(A)          0             |
>>    qdisc_destroy(A)                            |
>>    tcf_chain0_head_change_cb_del(A)            |
>>  ! mini_qdisc_pair_swap(A)                     |
>
>   3. A destroys itself,
>      miniq_old (eth0->miniq_ingress) is A's mini Qdisc,
>      (destroying, so no RCU wait)
>      change eth0->miniq_ingress to NULL,
>      update miniq_old, or A's mini Qdisc's RCU state,
>      A is freed.
>
>                       2. RCU wait ends,
> 		         change eth0->miniq_ingress to B's mini Qdisc,
> 	 use-after-free: update miniq_old, or A's mini Qdisc's RCU state.

Thanks for the clarification.

>
> I hope this helps.  Sorry I didn't go into details; this UAF isn't the
> only thing that is unacceptable here:
>
> Consider B.  We add a filter Y to B, expecting ingress packets on eth0
> to go through Y.  Then all of a sudden, A sets eth0->miniq_ingress to
> NULL during its destruction, so packets will not find Y at all on
> datapath (sch_handle_ingress()).  New filter becomes invisible - this is
> already buggy enough :-/
>
> So I think B's first call to mini_qdisc_pair_swap() should happen after
> A's last call (in ingress_destroy()), which is what I am trying to
> achieve here.

Makes sense to me.