kernel-hardening - Re: [RFC PATCH v3 1/8] Use refcount

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Date: Mon, 18 Jan 2021 21:56:29 +0100
From: Alexey Gladkov <gladkov.alexey@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	io-uring <io-uring@...r.kernel.org>,
	Kernel Hardening <kernel-hardening@...ts.openwall.com>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Brauner <christian.brauner@...ntu.com>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
	Kees Cook <keescook@...omium.org>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting

On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote:
> On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov
> <gladkov.alexey@...il.com> wrote:
> >
> > Sorry about that. I thought that this code is not needed when switching
> > from int to refcount_t. I was wrong.
> 
> Well, you _may_ be right. I personally didn't check how the return
> value is used.
> 
> I only reacted to "it certainly _may_ be used, and there is absolutely
> no comment anywhere about why it wouldn't matter".

I have not found examples where checked the overflow after calling
refcount_inc/refcount_add.

For example in kernel/fork.c:2298 :

   current->signal->nr_threads++;                           
   atomic_inc(&current->signal->live);                      
   refcount_inc(&current->signal->sigcnt);  

$ semind search signal_struct.sigcnt
def include/linux/sched/signal.h:83  		refcount_t		sigcnt;
m-- kernel/fork.c:723 put_signal_struct 		if (refcount_dec_and_test(&sig->sigcnt))
m-- kernel/fork.c:1571 copy_signal 		refcount_set(&sig->sigcnt, 1);
m-- kernel/fork.c:2298 copy_process 				refcount_inc(&current->signal->sigcnt);

It seems to me that the only way is to use __refcount_inc and then compare
the old value with REFCOUNT_MAX

Since I have not seen examples of such checks, I thought that this is
acceptable. Sorry once again. I have not tried to hide these changes.

-- 
Rgrds, legion

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