RE: [PATCH] selinux: convert struct sidtab count to refcount_t

Gote, Nitin R
2019-07-23T05:44:41Z

> -----Original Message-----
> From: Ondrej Mosnacek [mailto:omosnace@redhat.com]
> Sent: Monday, July 22, 2019 6:48 PM
> To: Gote, Nitin R <nitin.r.gote@intel.com>
> Cc: Kees Cook <keescook@chromium.org>; 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
>
> 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.

Agreed. Thanks for clarification.
I'm going to discontinue this patch.

>
> Your patch, however, lead me to realize that I forgot to guard against
> overflow above SIDTAB_MAX when a new entry is being inserted. It is
> extremely unlikely to happen in practice, but should be fixed anyway.
> I'll send a patch shortly.
>

Thank you.

> > ---
> > security/selinux/ss/sidtab.c | 16 ++++++++--------
> > security/selinux/ss/sidtab.h | 2 +-
> > 2 files changed, 9 insertions(+), 9 deletions(-)
> >
> > diff --git a/security/selinux/ss/sidtab.c
> > b/security/selinux/ss/sidtab.c index e63a90ff2728..20fe235c6c71 100644
> > --- a/security/selinux/ss/sidtab.c
> > +++ b/security/selinux/ss/sidtab.c
> > @@ -29,7 +29,7 @@ int sidtab_init(struct sidtab *s)
> > for (i = 0; i < SECINITSID_NUM; i++)
> > s->isids[i].set = 0;
> >
> > - atomic_set(&s->count, 0);
> > + refcount_set(&s->count, 0);
> >
> > s->convert = NULL;
> >
> > @@ -130,7 +130,7 @@ static struct context *sidtab_do_lookup(struct
> > sidtab *s, u32 index, int alloc)
> >
> > static struct context *sidtab_lookup(struct sidtab *s, u32 index) {
> > - u32 count = (u32)atomic_read(&s->count);
> > + u32 count = refcount_read(&s->count);
> >
> > if (index >= count)
> > return NULL;
> > @@ -245,7 +245,7 @@ static int sidtab_reverse_lookup(struct sidtab *s,
> struct context *context,
> > u32 *index)
> > {
> > unsigned long flags;
> > - u32 count = (u32)atomic_read(&s->count);
> > + u32 count = (u32)refcount_read(&s->count);
> > u32 count_locked, level, pos;
> > struct sidtab_convert_params *convert;
> > struct context *dst, *dst_convert;
> > @@ -272,7 +272,7 @@ static int sidtab_reverse_lookup(struct sidtab *s,
> struct context *context,
> > spin_lock_irqsave(&s->lock, flags);
> >
> > convert = s->convert;
> > - count_locked = (u32)atomic_read(&s->count);
> > + count_locked = (u32)refcount_read(&s->count);
> > level = sidtab_level_from_count(count_locked);
> >
> > /* if count has changed before we acquired the lock, then catch up */
> > @@ -315,7 +315,7 @@ static int sidtab_reverse_lookup(struct sidtab *s,
> struct context *context,
> > }
> >
> > /* at this point we know the insert won't fail */
> > - atomic_set(&convert->target->count, count + 1);
> > + refcount_set(&convert->target->count, count + 1);
> > }
> >
> > if (context->len)
> > @@ -328,7 +328,7 @@ static int sidtab_reverse_lookup(struct sidtab *s,
> struct context *context,
> > /* write entries before writing new count */
> > smp_wmb();
> >
> > - atomic_set(&s->count, count + 1);
> > + refcount_set(&s->count, count + 1);
> >
> > rc = 0;
> > out_unlock:
> > @@ -418,7 +418,7 @@ int sidtab_convert(struct sidtab *s, struct
> sidtab_convert_params *params)
> > return -EBUSY;
> > }
> >
> > - count = (u32)atomic_read(&s->count);
> > + count = (u32)refcount_read(&s->count);
> > level = sidtab_level_from_count(count);
> >
> > /* allocate last leaf in the new sidtab (to avoid race with
> > @@ -431,7 +431,7 @@ int sidtab_convert(struct sidtab *s, struct
> sidtab_convert_params *params)
> > }
> >
> > /* set count in case no new entries are added during conversion */
> > - atomic_set(&params->target->count, count);
> > + refcount_set(&params->target->count, count);
> >
> > /* enable live convert of new entries */
> > s->convert = params;
> > diff --git a/security/selinux/ss/sidtab.h b/security/selinux/ss/sidtab.h
> > index bbd5c0d1f3bd..68dd96a5beba 100644
> > --- a/security/selinux/ss/sidtab.h
> > +++ b/security/selinux/ss/sidtab.h
> > @@ -70,7 +70,7 @@ struct sidtab_convert_params {
> >
> > struct sidtab {
> > union sidtab_entry_inner roots[SIDTAB_MAX_LEVEL + 1];
> > - atomic_t count;
> > + refcount_t count;
> > struct sidtab_convert_params *convert;
> > spinlock_t lock;
> >
> > --
> > 2.17.1
> >
>
> Thanks,
>
> --
> Ondrej Mosnacek <omosnace at redhat dot com>
> Software Engineer, Security Technologies
> Red Hat, Inc.