linux-kernel - Re: [PATCH RFC] cred: Add WARN to detect wrong use of get/put

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Message-ID: <202006121001.149B5D20C5@keescook>
Date:   Fri, 12 Jun 2020 10:06:49 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Xiaoming Ni <nixiaoming@...wei.com>, paul@...l-moore.com,
        edumazet@...gle.com, paulmck@...nel.org, dhowells@...hat.com,
        shakeelb@...gle.com, jamorris@...ux.microsoft.com,
        alex.huangjianhui@...wei.com, dylix.dailei@...wei.com,
        chenzefeng2@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] cred: Add WARN to detect wrong use of get/put_cred

On Fri, Jun 12, 2020 at 06:33:45PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 12, 2020 at 06:28:15PM +0800, Xiaoming Ni wrote:
> > Cred release and usage check code flow:
> > 	1. put_cred()
> > 		if (atomic_dec_and_test(&(cred)->usage))
> > 			__put_cred(cred);
> > 
> > 	2. __put_cred()
> > 		BUG_ON(atomic_read(&cred->usage) != 0);
> > 		call_rcu(&cred->rcu, put_cred_rcu);
> > 
> > 	3. put_cred_rcu()
> > 		if (atomic_read(&cred->usage) != 0)
> > 			panic("CRED: put_cred_rcu() sees %p with usage %d\n",
> > 			       cred, atomic_read(&cred->usage));
> > 		kmem_cache_free(cred_jar, cred);
> > 
> > If panic is triggered on put_cred_rcu(), there are two possibilities
> > 	1. Call get_cred() after __put_cred(), usage > 0
> > 	2. Call put_cred() after __put_cred(), usage < 0
> > Since put_cred_rcu is an asynchronous behavior, it is no longer the first
> > scene when panic, there is no information about the murderer in the panic
> > call stack...
> > 
> > So, add WARN() in get_cred()/put_cred(), and pray to catch the murderer
> > at the first scene.
> 
> Why not not use refcount_t? It has all that goodness and more.

I thought these had been applied already, I guess not:
https://lore.kernel.org/lkml/20190306110549.7628-1-elena.reshetova@intel.com/

Can we try again?

-- 
Kees Cook