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

[Gote, Nitin R]

> -----Original Message-----
> From: Ondrej Mosnacek [mailto:omosnace@xxxxxxxxxx]
> Sent: Monday, July 22, 2019 6:48 PM
> To: Gote, Nitin R <nitin.r.gote@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>; kernel-
> hardening@xxxxxxxxxxxxxxxxxx; Paul Moore <paul@xxxxxxxxxxxxxx>;
> Stephen Smalley <sds@xxxxxxxxxxxxx>; Eric Paris <eparis@xxxxxxxxxxxxxx>;
> SElinux list <selinux@xxxxxxxxxxxxxxx>; Linux kernel mailing list <linux-
> kernel@xxxxxxxxxxxxxxx>
> Subject: Re: [PATCH] selinux: convert struct sidtab count to refcount_t
> 
> On Mon, Jul 22, 2019 at 1:35 PM NitinGote <nitin.r.gote@xxxxxxxxx> 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@xxxxxxxxx>
> 
> 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.