From: Jann Horn <jannh@google.com>
To: Ondrej Mosnacek <omosnace@redhat.com>
Cc: NitinGote <nitin.r.gote@intel.com>,
Kees Cook <keescook@chromium.org>,
Kernel Hardening <kernel-hardening@lists.openwall.com>,
Paul Moore <paul@paul-moore.com>,
Stephen Smalley <sds@tycho.nsa.gov>,
Eric Paris <eparis@parisplace.org>,
SElinux list <selinux@vger.kernel.org>,
Linux kernel mailing list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] selinux: convert struct sidtab count to refcount_t
Date: Tue, 23 Jul 2019 16:53:47 +0200 [thread overview]
Message-ID: <CAG48ez3zRoB7awMdb-koKYJyfP9WifTLevxLxLHioLhH=itZ-A@mail.gmail.com> (raw)
In-Reply-To: <CAFqZXNs5vdQwoy2k=_XLiGRdyZCL=n8as6aL01Dw-U62amFREA@mail.gmail.com>
On Mon, Jul 22, 2019 at 3:44 PM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> On Mon, Jul 22, 2019 at 1:35 PM NitinGote <nitin.r.gote@intel.com> wrote:
> > refcount_t type and corresponding API should be
> > used instead of atomic_t when the variable is used as
> > a reference counter. This allows to avoid accidental
> > refcounter overflows that might lead to use-after-free
> > situations.
> >
> > Signed-off-by: NitinGote <nitin.r.gote@intel.com>
>
> Nack.
>
> The 'count' variable is not used as a reference counter here. It
> tracks the number of entries in sidtab, which is a very specific
> lookup table that can only grow (the count never decreases). I only
> made it atomic because the variable is read outside of the sidtab's
> spin lock and thus the reads and writes to it need to be guaranteed to
> be atomic. The counter is only updated under the spin lock, so
> insertions do not race with each other.
Probably shouldn't even be atomic_t... quoting Documentation/atomic_t.txt:
| SEMANTICS
| ---------
|
| Non-RMW ops:
|
| The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
| implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
| smp_store_release() respectively. Therefore, if you find yourself only using
| the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
| and are doing it wrong.
So I think what you actually want here is a plain "int count", and then:
- for unlocked reads, either READ_ONCE()+smp_rmb() or smp_load_acquire()
- for writes, either smp_wmb()+WRITE_ONCE() or smp_store_release()
smp_load_acquire() and smp_store_release() are probably the nicest
here, since they are semantically clearer than smp_rmb()/smp_wmb().
next prev parent reply other threads:[~2019-07-23 14:54 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-22 11:31 [PATCH] selinux: convert struct sidtab count to refcount_t NitinGote
2019-07-22 13:17 ` Ondrej Mosnacek
2019-07-23 0:25 ` Paul Moore
2019-07-23 5:44 ` Gote, Nitin R
2019-07-23 14:53 ` Jann Horn [this message]
2019-07-23 22:17 ` Kees Cook
2019-07-24 14:28 ` Jann Horn
2019-07-24 15:54 ` Kees Cook
2019-07-24 16:55 ` Jann Horn
2019-07-29 16:51 ` Kees Cook
2019-07-24 16:17 ` Ondrej Mosnacek
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