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Date:   Tue, 1 Sep 2020 12:18:24 +0000
From:   Tuong Tong Lien <tuong.t.lien@...tech.com.au>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "jmaloy@...hat.com" <jmaloy@...hat.com>,
        "maloy@...jonn.com" <maloy@...jonn.com>,
        "ying.xue@...driver.com" <ying.xue@...driver.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "tipc-discussion@...ts.sourceforge.net" 
        <tipc-discussion@...ts.sourceforge.net>
Subject: RE: [net] tipc: fix using smp_processor_id() in preemptible



> -----Original Message-----
> From: Eric Dumazet <eric.dumazet@...il.com>
> Sent: Monday, August 31, 2020 7:48 PM
> To: Tuong Tong Lien <tuong.t.lien@...tech.com.au>; Eric Dumazet <eric.dumazet@...il.com>; davem@...emloft.net;
> jmaloy@...hat.com; maloy@...jonn.com; ying.xue@...driver.com; netdev@...r.kernel.org
> Cc: tipc-discussion@...ts.sourceforge.net
> Subject: Re: [net] tipc: fix using smp_processor_id() in preemptible
> 
> 
> 
> On 8/31/20 3:05 AM, Tuong Tong Lien wrote:
> >
> >
> >> -----Original Message-----
> >> From: Eric Dumazet <eric.dumazet@...il.com>
> >> Sent: Monday, August 31, 2020 4:48 PM
> >> To: Tuong Tong Lien <tuong.t.lien@...tech.com.au>; Eric Dumazet <eric.dumazet@...il.com>; davem@...emloft.net;
> >> jmaloy@...hat.com; maloy@...jonn.com; ying.xue@...driver.com; netdev@...r.kernel.org
> >> Cc: tipc-discussion@...ts.sourceforge.net
> >> Subject: Re: [net] tipc: fix using smp_processor_id() in preemptible
> >>
> >>
> >>
> >> On 8/31/20 1:33 AM, Tuong Tong Lien wrote:
> >>> Hi Eric,
> >>>
> >>> Thanks for your comments, please see my answers inline.
> >>>
> >>>> -----Original Message-----
> >>>> From: Eric Dumazet <eric.dumazet@...il.com>
> >>>> Sent: Monday, August 31, 2020 3:15 PM
> >>>> To: Tuong Tong Lien <tuong.t.lien@...tech.com.au>; davem@...emloft.net; jmaloy@...hat.com; maloy@...jonn.com;
> >>>> ying.xue@...driver.com; netdev@...r.kernel.org
> >>>> Cc: tipc-discussion@...ts.sourceforge.net
> >>>> Subject: Re: [net] tipc: fix using smp_processor_id() in preemptible
> >>>>
> >>>>
> >>>>
> >>>> On 8/29/20 12:37 PM, Tuong Lien wrote:
> >>>>> The 'this_cpu_ptr()' is used to obtain the AEAD key' TFM on the current
> >>>>> CPU for encryption, however the execution can be preemptible since it's
> >>>>> actually user-space context, so the 'using smp_processor_id() in
> >>>>> preemptible' has been observed.
> >>>>>
> >>>>> We fix the issue by using the 'get/put_cpu_ptr()' API which consists of
> >>>>> a 'preempt_disable()' instead.
> >>>>>
> >>>>> Fixes: fc1b6d6de220 ("tipc: introduce TIPC encryption & authentication")
> >>>>
> >>>> Have you forgotten ' Reported-by: syzbot+263f8c0d007dc09b2dda@...kaller.appspotmail.com' ?
> >>> Well, really I detected the issue during my testing instead, didn't know if it was reported by syzbot too.
> >>>
> >>>>
> >>>>> Acked-by: Jon Maloy <jmaloy@...hat.com>
> >>>>> Signed-off-by: Tuong Lien <tuong.t.lien@...tech.com.au>
> >>>>> ---
> >>>>>  net/tipc/crypto.c | 12 +++++++++---
> >>>>>  1 file changed, 9 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/net/tipc/crypto.c b/net/tipc/crypto.c
> >>>>> index c38babaa4e57..7c523dc81575 100644
> >>>>> --- a/net/tipc/crypto.c
> >>>>> +++ b/net/tipc/crypto.c
> >>>>> @@ -326,7 +326,8 @@ static void tipc_aead_free(struct rcu_head *rp)
> >>>>>  	if (aead->cloned) {
> >>>>>  		tipc_aead_put(aead->cloned);
> >>>>>  	} else {
> >>>>> -		head = *this_cpu_ptr(aead->tfm_entry);
> >>>>> +		head = *get_cpu_ptr(aead->tfm_entry);
> >>>>> +		put_cpu_ptr(aead->tfm_entry);
> >>>>
> >>>> Why is this safe ?
> >>>>
> >>>> I think that this very unusual construct needs a comment, because this is not obvious.
> >>>>
> >>>> This really looks like an attempt to silence syzbot to me.
> >>> No, this is not to silence syzbot but really safe.
> >>> This is because the "aead->tfm_entry" object is "common" between CPUs, there is only its pointer to be the "per_cpu" one. So
> just
> >> trying to lock the process on the current CPU or 'preempt_disable()', taking the per-cpu pointer and dereferencing to the actual
> >> "tfm_entry" object... is enough. Later on, that’s fine to play with the actual object without any locking.
> >>
> >> Why using per cpu pointers, if they all point to a common object ?
> >>
> >> This makes the code really confusing.
> > Sorry for making you confused. Yes, the code is a bit ugly and could be made in some other ways... The initial idea is to not touch or
> change the same pointer variable in different CPUs so avoid a penalty with the cache hits/misses...
> 
> What makes this code interrupt safe ?
> 
Why is it unsafe? Its "parent" object is already managed by RCU mechanism. Also, it is never modified but just "read-only" in all cases...

BR/Tuong
> Having a per-cpu list is not interrupt safe without special care.
> 
> 

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