Date: Tue, 23 Jul 2019 15:17:49 -0700 From: Kees Cook <keescook@...omium.org> To: Jann Horn <jannh@...gle.com> Cc: Ondrej Mosnacek <omosnace@...hat.com>, NitinGote <nitin.r.gote@...el.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Paul Moore <paul@...l-moore.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 Tue, Jul 23, 2019 at 04:53:47PM +0200, Jann Horn wrote: > On Mon, Jul 22, 2019 at 3:44 PM 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. > > 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(). Perhaps we need a "statistics" counter type for these kinds of counters? "counter_t"? I bet there are a lot of atomic_t uses that are just trying to be counters. (likely most of atomic_t that isn't now refcount_t ...) -- Kees Cook
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