netdev - RE: [net] tipc: fix using smp_processor

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Date:   Mon, 31 Aug 2020 08:33:02 +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

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.

BR/Tuong
> 
> >  		list_for_each_entry_safe(tfm_entry, tmp, &head->list, list) {
> >  			crypto_free_aead(tfm_entry->tfm);
> >  			list_del(&tfm_entry->list);
> > @@ -399,10 +400,15 @@ static void tipc_aead_users_set(struct tipc_aead __rcu *aead, int val)
> >   */
> >  static struct crypto_aead *tipc_aead_tfm_next(struct tipc_aead *aead)
> >  {
> > -	struct tipc_tfm **tfm_entry = this_cpu_ptr(aead->tfm_entry);
> > +	struct tipc_tfm **tfm_entry;
> > +	struct crypto_aead *tfm;
> >
> > +	tfm_entry = get_cpu_ptr(aead->tfm_entry);
> >  	*tfm_entry = list_next_entry(*tfm_entry, list);
> > -	return (*tfm_entry)->tfm;
> > +	tfm = (*tfm_entry)->tfm;
> > +	put_cpu_ptr(tfm_entry);
> 
> Again, this looks suspicious to me. I can not explain why this would be safe.
> 
> > +
> > +	return tfm;
> >  }
> >
> >  /**
> >