> -----Original Message-----
> From: Eric Dumazet <eric.duma...@gmail.com>
> Sent: Monday, August 31, 2020 7:48 PM
> To: Tuong Tong Lien <tuong.t.l...@dektech.com.au>; Eric Dumazet
> <eric.duma...@gmail.com>; da...@davemloft.net;
> jma...@redhat.com; ma...@donjonn.com; ying....@windriver.com;
> netdev@vger.kernel.org
> Cc: tipc-discuss...@lists.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.duma...@gmail.com>
> >> Sent: Monday, August 31, 2020 4:48 PM
> >> To: Tuong Tong Lien <tuong.t.l...@dektech.com.au>; Eric Dumazet
> >> <eric.duma...@gmail.com>; da...@davemloft.net;
> >> jma...@redhat.com; ma...@donjonn.com; ying....@windriver.com;
> >> netdev@vger.kernel.org
> >> Cc: tipc-discuss...@lists.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.duma...@gmail.com>
> >>>> Sent: Monday, August 31, 2020 3:15 PM
> >>>> To: Tuong Tong Lien <tuong.t.l...@dektech.com.au>; da...@davemloft.net;
> >>>> jma...@redhat.com; ma...@donjonn.com;
> >>>> ying....@windriver.com; netdev@vger.kernel.org
> >>>> Cc: tipc-discuss...@lists.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+263f8c0d007dc09b2...@syzkaller.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 <jma...@redhat.com>
> >>>>> Signed-off-by: Tuong Lien <tuong.t.l...@dektech.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.
>
>
