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

From: Peilin Ye <[email-masked]>
Date: Wed, 26 Apr 2023 16:42:01 -0700

+Cc: Vlad Buslov, Hillf Danton

Hi all,

On Mon, Apr 17, 2023 at 04:00:11PM -0700, Peilin Ye wrote:
> I also reproduced this UAF using the syzkaller reproducer in the report
> (the C reproducer did not work for me for unknown reasons).  I will look
> into this.

Currently, multiple ingress (clsact) Qdiscs can access the per-netdev
*miniq_ingress (*miniq_egress) pointer concurrently.  This is
unfortunately true in two senses:

1. We allow adding ingress (clsact) Qdiscs under parents other than
TC_H_INGRESS (TC_H_CLSACT):

  $ ip link add ifb0 numtxqueues 8 type ifb
  $ echo clsact > /proc/sys/net/core/default_qdisc
  $ tc qdisc add dev ifb0 handle 1: root mq
  $ tc qdisc show dev ifb0
  qdisc mq 1: root
  qdisc clsact 0: parent 1:8
  qdisc clsact 0: parent 1:7
  qdisc clsact 0: parent 1:6
  qdisc clsact 0: parent 1:5
  qdisc clsact 0: parent 1:4
  qdisc clsact 0: parent 1:3
  qdisc clsact 0: parent 1:2
  qdisc clsact 0: parent 1:1

This is obviously racy and should be prohibited.  I've started working
on patches to fix this.  The syz repro for this UAF adds ingress Qdiscs
under TC_H_ROOT, by the way.

2. After introducing RTNL-lockless RTM_{NEW,DEL,GET}TFILTER requests
[1], it is possible that, when replacing ingress (clsact) Qdiscs, the
old one can access *miniq_{in,e}gress concurrently with the new one.  For
example, the syz repro does something like the following:

  Thread 1 creates sch_ingress Qdisc A (containing mini Qdisc a1 and a2),
  then adds a cls_flower filter X to Qdisc A.

  Thread 2 creates sch_ingress Qdisc B (containing mini Qdisc b1 and b2)
  to replace Qdisc A, then adds a cls_flower filter Y to Qdisc B.

  Device has 8 TXQs.

 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)           
            |                        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)
   tcf_block_release(A)          0             |
   qdisc_destroy(A)                            |
   tcf_chain0_head_change_cb_del(A)            |
 ! mini_qdisc_pair_swap(A)                     |
            |                                  |
           ...                                ...

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.

To fix this, I'm cooking a patch that, when replacing ingress (clsact)
Qdiscs, in qdisc_graft():

  I.  We should make sure there's no on-the-fly lockless filter requests
      for the old Qdisc, and return -EBUSY if there's any (or can/should
      we wait in RTM_NEWQDISC handler?)

  II. We should destory the old Qdisc before publishing the new one
      (i.e. setting it to dev_ingress_queue(dev)->qdisc_sleeping, so
      that subsequent filter requests can see it), because
      {ingress,clsact}_destroy() also call mini_qdisc_pair_swap(), which
      sets *miniq_{in,e}gress to NULL

Future Qdiscs that support RTNL-lockless cls_ops, if any, won't need
this fix, as long as their ->chain_head_change() don't access
out-of-Qdisc-scope data, like pointers in struct net_device.

Do you think this is the right way to go?  Thanks!

[1] Thanks Hillf Danton for the hint:
    https://syzkaller.appspot.com/text?tag=Patch&x=10d7cd5bc80000

Thanks,
Peilin Ye