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From: Kaiwan N Billimoria <[email protected]>
To: Alexey Gladkov <[email protected]>
Cc: Linus Torvalds <[email protected]>,
	LKML <[email protected]>,
	io-uring <[email protected]>,
	Kernel Hardening <[email protected]>,
	Linux Containers <[email protected]>,
	Linux-MM <[email protected]>,
	Andrew Morton <[email protected]>,
	Christian Brauner <[email protected]>,
	"Eric W . Biederman" <[email protected]>,
	Jann Horn <[email protected]>, Jens Axboe <[email protected]>,
	Kees Cook <[email protected]>,
	Oleg Nesterov <[email protected]>
Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting
Date: Tue, 19 Jan 2021 10:05:03 +0530	[thread overview]
Message-ID: <CAPDLWs-fefTqAe+z-7BeALFpinanfPPd-9rmjKwUQ6WRP3_1Tg@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>

(Sorry for the gmail client)
My 0.2, HTH:
a) AFAIK, refcount_inc() (and similar friends) don't return any value
b) they're designed to just WARN() if they saturate or if you're
attempting to increment the value 0 (as it's possibly a UAF bug)
c) refcount_inc_checked() is documented as "Similar to atomic_inc(),
but will saturate at UINT_MAX and WARN"
d) we should avoid using the __foo() when foo() 's present as far as
is sanely possible...

So is one expected to just fix things when they break? - as signalled
by the WARN firing?

--
Regards, kaiwan.


On Tue, Jan 19, 2021 at 2:26 AM Alexey Gladkov <[email protected]> wrote:
>
> On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote:
> > On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov
> > <[email protected]> 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
>
>

  reply	other threads:[~2021-01-19  5:42 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-15 14:57 [RFC PATCH v3 0/8] Count rlimits in each user namespace Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting Alexey Gladkov
2021-01-18 19:14   ` Linus Torvalds
2021-01-18 19:45     ` Alexey Gladkov
2021-01-18 20:34       ` Linus Torvalds
2021-01-18 20:56         ` Alexey Gladkov
2021-01-19  4:35           ` Kaiwan N Billimoria [this message]
2021-01-20  1:57           ` Eric W. Biederman
2021-01-20  1:58             ` Eric W. Biederman
2021-01-21 12:04             ` Alexey Gladkov
2021-01-21 15:50               ` Eric W. Biederman
2021-01-21 16:07                 ` Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 2/8] Add a reference to ucounts for each cred Alexey Gladkov
2021-01-18  8:31   ` [PATCH v4 " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 3/8] Move RLIMIT_NPROC counter to ucounts Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 4/8] Move RLIMIT_MSGQUEUE " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 5/8] Move RLIMIT_SIGPENDING " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 6/8] Move RLIMIT_MEMLOCK " Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 7/8] Move RLIMIT_NPROC check to the place where we increment the counter Alexey Gladkov
2021-01-15 14:57 ` [RFC PATCH v3 8/8] kselftests: Add test to check for rlimit changes in different user namespaces Alexey Gladkov

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