Re: [PATCH net v4] cls_u32: fix use after free in u32_destroy_key()

From: Paolo Abeni <pabeni@redhat.com>
Date: Mon,  5 Feb 2018 22:23:01 +0100

Li Shuang reported an Oops with cls_u32 due to an use-after-free
in u32_destroy_key(). The use-after-free can be triggered with:

dev=lo
tc qdisc add dev $dev root handle 1: htb default 10
tc filter add dev $dev parent 1: prio 5 handle 1: protocol ip u32 divisor 256
tc filter add dev $dev protocol ip parent 1: prio 5 u32 ht 800:: match ip dst\
 10.0.0.0/8 hashkey mask 0x0000ff00 at 16 link 1:
tc qdisc del dev $dev root

Which causes the following kasan splat:

 ...

The problem is that the htnode is freed before the linked knodes and the
latter will try to access the first at u32_destroy_key() time.
This change addresses the issue using the htnode refcnt to guarantee
the correct free order. While at it also add a RCU annotation,
to keep sparse happy.

v1 -> v2: use rtnl_derefence() instead of RCU read locks
v2 -> v3:
  - don't check refcnt in u32_destroy_hnode()
  - cleaned-up u32_destroy() implementation
  - cleaned-up code comment
v3 -> v4:
  - dropped unneeded comment

Reported-by: Li Shuang <redacted>
Fixes: c0d378ef1266 ("net_sched: use tcf_queue_work() in u32 filter")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Applied and queued up for -stable, thanks!