Re: [PATCH RFC] cred: Add WARN to detect wrong use of get/put_cred

[Kees Cook]

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@xxxxxxxxx/

Can we try again?

-- 
Kees Cook