From: "Gote, Nitin R" <nitin.r.gote@intel.com>
To: Ondrej Mosnacek <omosnace@redhat.com>
Cc: Kees Cook <keescook@chromium.org>,
"kernel-hardening@lists.openwall.com"
<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 05:44:30 +0000 [thread overview]
Message-ID: <12356C813DFF6F479B608F81178A561587AAD1@BGSMSX101.gar.corp.intel.com> (raw)
In-Reply-To: <CAFqZXNs5vdQwoy2k=_XLiGRdyZCL=n8as6aL01Dw-U62amFREA@mail.gmail.com>
> -----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(¶ms->target->count, count);
> > + refcount_set(¶ms->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.
next prev parent reply other threads:[~2019-07-23 5:44 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 [this message]
2019-07-23 14:53 ` Jann Horn
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=12356C813DFF6F479B608F81178A561587AAD1@BGSMSX101.gar.corp.intel.com \
--to=nitin.r.gote@intel.com \
--cc=eparis@parisplace.org \
--cc=keescook@chromium.org \
--cc=kernel-hardening@lists.openwall.com \
--cc=linux-kernel@vger.kernel.org \
--cc=omosnace@redhat.com \
--cc=paul@paul-moore.com \
--cc=sds@tycho.nsa.gov \
--cc=selinux@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).