linux-kernel - Re: [PATCH] selinux: convert struct sidtab count to refcount

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Date:   Mon, 22 Jul 2019 20:25:18 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Ondrej Mosnacek <omosnace@...hat.com>,
        NitinGote <nitin.r.gote@...el.com>
Cc:     Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        SElinux list <selinux@...r.kernel.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] selinux: convert struct sidtab count to refcount_t

On Mon, Jul 22, 2019 at 9:18 AM Ondrej Mosnacek <omosnace@...hat.com> wrote:
> On Mon, Jul 22, 2019 at 1:35 PM NitinGote <nitin.r.gote@...el.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@...el.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.

Agreed, this should be changed to use refcount_t.

-- 
paul moore
www.paul-moore.com