[PATCH v4 1/9] capability: introduce new capable flag NODENYAUDIT

[Paul Moore]

On Wed, May 31, 2023 at 2:34 PM Christian Göttsche
<cgzones at googlemail.com> wrote:
> On Wed, 31 May 2023 at 16:08, Serge E. Hallyn <serge at hallyn.com> wrote:
> >
> > On Wed, May 31, 2023 at 09:07:34AM -0500, Serge E. Hallyn wrote:
> > > On Thu, May 11, 2023 at 04:25:24PM +0200, Christian Göttsche wrote:
> > > > Introduce a new capable flag, CAP_OPT_NODENYAUDIT, to not generate
> > > > an audit event if the requested capability is not granted.  This will be
> > > > used in a new capable_any() functionality to reduce the number of
> > > > necessary capable calls.
> > > >
> > > > Handle the flag accordingly in AppArmor and SELinux.
> > > >
> > > > Suggested-by: Paul Moore <paul at paul-moore.com>
> > > > Signed-off-by: Christian Göttsche <cgzones at googlemail.com>
> > >
> > > Reviewed-by: Serge Hallyn <serge at hallyn.com>
> >
> > Sorry, obviously I should have removed this, until the comment below was
> > answered :)
> >
> > > > ---
> > > >  include/linux/security.h       |  2 ++
> > > >  security/apparmor/capability.c |  8 +++++---
> > > >  security/selinux/hooks.c       | 14 ++++++++------
> > > >  3 files changed, 15 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/include/linux/security.h b/include/linux/security.h
> > > > index e2734e9e44d5..629c775ec297 100644
> > > > --- a/include/linux/security.h
> > > > +++ b/include/linux/security.h
> > > > @@ -67,6 +67,8 @@ struct watch_notification;
> > > >  #define CAP_OPT_NOAUDIT BIT(1)
> > > >  /* If capable is being called by a setid function */
> > > >  #define CAP_OPT_INSETID BIT(2)
> > > > +/* If capable should audit the security request for authorized requests only */
> > > > +#define CAP_OPT_NODENYAUDIT BIT(3)
> > > >
> > > >  /* LSM Agnostic defines for security_sb_set_mnt_opts() flags */
> > > >  #define SECURITY_LSM_NATIVE_LABELS 1
> > > > diff --git a/security/apparmor/capability.c b/security/apparmor/capability.c
> > > > index 326a51838ef2..98120dd62ca7 100644
> > > > --- a/security/apparmor/capability.c
> > > > +++ b/security/apparmor/capability.c
> > > > @@ -108,7 +108,8 @@ static int audit_caps(struct common_audit_data *sa, struct aa_profile *profile,
> > > >   * profile_capable - test if profile allows use of capability @cap
> > > >   * @profile: profile being enforced    (NOT NULL, NOT unconfined)
> > > >   * @cap: capability to test if allowed
> > > > - * @opts: CAP_OPT_NOAUDIT bit determines whether audit record is generated
> > > > + * @opts: CAP_OPT_NOAUDIT/CAP_OPT_NODENYAUDIT bit determines whether audit
> > > > + * record is generated
> > > >   * @sa: audit data (MAY BE NULL indicating no auditing)
> > > >   *
> > > >   * Returns: 0 if allowed else -EPERM
> > > > @@ -126,7 +127,7 @@ static int profile_capable(struct aa_profile *profile, int cap,
> > > >     else
> > > >             error = -EPERM;
> > > >
> > > > -   if (opts & CAP_OPT_NOAUDIT) {
> > > > +   if ((opts & CAP_OPT_NOAUDIT) || ((opts & CAP_OPT_NODENYAUDIT) && error)) {
> > > >             if (!COMPLAIN_MODE(profile))
> > > >                     return error;
> > > >             /* audit the cap request in complain mode but note that it
> > > > @@ -142,7 +143,8 @@ static int profile_capable(struct aa_profile *profile, int cap,
> > > >   * aa_capable - test permission to use capability
> > > >   * @label: label being tested for capability (NOT NULL)
> > > >   * @cap: capability to be tested
> > > > - * @opts: CAP_OPT_NOAUDIT bit determines whether audit record is generated
> > > > + * @opts: CAP_OPT_NOAUDIT/CAP_OPT_NODENYAUDIT bit determines whether audit
> > > > + * record is generated
> > > >   *
> > > >   * Look up capability in profile capability set.
> > > >   *
> > > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > > > index 79b4890e9936..0730edf2f5f1 100644
> > > > --- a/security/selinux/hooks.c
> > > > +++ b/security/selinux/hooks.c
> > > > @@ -1571,7 +1571,7 @@ static int cred_has_capability(const struct cred *cred,
> > > >     u16 sclass;
> > > >     u32 sid = cred_sid(cred);
> > > >     u32 av = CAP_TO_MASK(cap);
> > > > -   int rc;
> > > > +   int rc, rc2;
> > > >
> > > >     ad.type = LSM_AUDIT_DATA_CAP;
> > > >     ad.u.cap = cap;
> > > > @@ -1590,11 +1590,13 @@ static int cred_has_capability(const struct cred *cred,
> > > >     }
> > > >
> > > >     rc = avc_has_perm_noaudit(sid, sid, sclass, av, 0, &avd);
> > > > -   if (!(opts & CAP_OPT_NOAUDIT)) {
> > > > -           int rc2 = avc_audit(sid, sid, sclass, av, &avd, rc, &ad);
> > > > -           if (rc2)
> > > > -                   return rc2;
> > > > -   }
> > > > +   if ((opts & CAP_OPT_NOAUDIT) || ((opts & CAP_OPT_NODENYAUDIT) && rc))
> > > > +           return rc;
> > >
> > > Hm, if the caller passes only CAP_OPT_NODENYAUDIT, and rc == 0, then
> > > you will audit the allow.  Is that what you want, or did you want, or
> > > did you want CAP_OPT_NODENYAUDIT to imply CAP_OPT_NOAUDIT?
> > >
>
> The new option should cause to issue an audit event if (and only if)
> the requested capability is in effect for the current task. If the
> task does not have the capability no audit event should be issued.
>
> The new option should not imply CAP_OPT_NOAUDIT since we want an audit
> event in the case the capability is in effect.
>
> I admit the naming is a bit confusing as CAP_OPT_NODENYAUDIT as well
> as the commit description contains a double negation (while the inline
> comment for the macro definition does not).
>
> Do you prefer naming the constant CAP_OPT_ALLOWAUDIT or CAP_OPT_AUDIT_ON_ALLOW?

I think we need a different name, although I'm struggling to think of
something ... I don't think ALLOWAUDIT is right, as I believe it
implies that it is needed to "allow" auditing to take place for the
operation.  AUDIT_ON_ALLOW is better, but it still seems like it would
be required if you wanted to generate audit records on a successful
operation, which isn't correct.  I think we need to focus on the idea
that the flag blocks auditing for denials.

CAP_OPT_NOAUDITDENY is pretty much what you have, but in my mind the
NOAUDITDENY shares enough with the existing NOAUDIT flag that it makes
a bit more sense.

I honestly don't know.  However, whatever you pick, make sure you
update patch 2/X so that the name of ns_capable_nodenyaudit() is kept
close to the flag's name.

-- 
paul-moore.com