netdev - Re: [PATCH net] net/sched: act_ife: initalize ife->metalist earlier

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Date:   Wed, 15 Jan 2020 10:14:19 +0100
From:   Davide Caratti <dcaratti@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] net/sched: act_ife: initalize ife->metalist earlier

On Tue, 2020-01-14 at 13:51 -0800, Eric Dumazet wrote:
> It seems better to init ife->metalist earlier in tcf_ife_init()
> to avoid the following crash :

hello Eric, and thanks for the patch.

If I well understand the problem, we have

_tcf_ife_cleanup()

that does dereference of NULL ife->metalist,
because it has not yet initialized by tcf_ife_init(). This happened
probably because the control action was not valid (hence the Fixes:tag):
so, tcf_ife_init() jumped to the error path before doing INIT_LIST_HEAD().

I applied your patch to my tree, and I see this:

net/sched/act_ife.c: In function 'tcf_ife_init':
net/sched/act_ife.c:533:3: warning: 'ife' may be used uninitialized in
this function [-Wmaybe-uninitialized]
   INIT_LIST_HEAD(&ife->metalist);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

And I think the warning is telling us a real problem, because

        ife = to_ife(*a);

is done below the if (!exists) { } statement where you are dereferencing
'ife'.

I think the proper fix should do one of these two things:
1) ensure that 'ife' is a valid pointer in the INIT_LIST_HEAD()
2) leave tcf_ife_init() as is, and fix the priblem in _tcf_ife_clenup() by
proper checking the value of ife->metalist  (which should be NULL in the
error path, because tcf_idr_create() does kzalloc() [1].
WDYT?

thanks!
-- 
davide

[1] https://elixir.bootlin.com/linux/latest/source/net/sched/act_api.c#L404