Subject: Re: [PATCH] net: sched: Fix memory exposure from short TCA_U32_SEL

From: Al Viro <[email-masked]>
Date: Fri, 31 Aug 2018 05:03:33 +0100

On Tue, Aug 28, 2018 at 04:59:38PM +0100, Al Viro wrote:
> On Tue, Aug 28, 2018 at 01:03:10AM +0100, Al Viro wrote:
> >                         if (tcf_exts_get_net(&n->exts))
> >                                 tcf_queue_work(&n->rwork, u32_delete_key_freepf_work);
> >                         else
> >                                 u32_destroy_key(n->tp, n, true);
> > ... and we hit u32_destroy_key(<tp>, <knode>, true), which does
> 
> Speaking of which, we'd better never hit that branch for other reasons - there's
> no RCU delay between removal of knode from the hash chain and its kfree().
> tcf_queue_work() does guarantee such delay (by use of queue_rcu_work()), direct
> call doesn't...
> 
> Anyway, whichever branch is taken, the memory corruption problem remains - the
> comments below are accurate, AFAICS.
> 
> > Incidentally, if we hit
> >                                 tcf_queue_work(&n->rwork, u32_delete_key_freepf_work);
> > instead of u32_destroy_key(), the things don't seem to be any better - we
> > won't do anything to <knode> until rtnl is dropped, so u32_destroy() won't
> > break on the second pass through the loop - it'll free <ht0> there and
> > return.  Setting us up for trouble, since when u32_delete_key_freepf_work()
> > finally gets to u32_destroy_key() we'll have <knode>->ht_down pointing
> > to freed memory and decrementing its contents...

Build the kernel with slab poisoning and try this:
tc qdisc add dev eth0 ingress
tc filter add dev eth0 parent ffff: protocol ip prio 100 handle 1: u32 divisor 1
tc filter add dev eth0 parent ffff: protocol ip prio 200 handle 2: u32 divisor 1
tc filter add dev eth0 parent ffff: protocol ip prio 100 handle 1:0:11 u32 ht 1: link 801: offset at 0 mask 0f00 shift 6 plus 0 eat match ip protocol 6 ff
tc filter delete dev eth0 parent ffff: protocol ip prio 200
tc filter change dev eth0 parent ffff: protocol ip prio 100 handle 1:0:11 u32 ht 1: link 0: offset at 0 mask 0f00 shift 6 plus 0 eat match ip protocol 6 ff
tc filter delete dev eth0 parent ffff: protocol ip prio 100

Then watch it oops in u32_lookup_ht() from u32_get() from tc_del_tfilter()
Oopsing insn: cmp    %ebp,0x8(%rbx).  RBX: 6b6b6b6b6b6b6b6b, i.e. slab
poison...

What happens is that ht 801: (created when we'd added tcf_proto for prio 200)
gets pinned down by link 801: in the third tc filter add.  Then removal of
prio 200 triggers u32_destroy(), dropping refcount on 801: and doing nothing
else to it.  Then filter change drops the last reference to 801:, freeing
it.  And we have a freed struct tc_u_hnode stuck in the middle of the list...