[PATCH] security: keys: perform capable check only on privileged operations

[Christian Göttsche]

On Fri, 19 May 2023 at 23:08, Paul Moore <paul at paul-moore.com> wrote:
>
> On Thu, May 11, 2023 at 8:33 AM Christian Göttsche
> <cgzones at googlemail.com> wrote:
> >
> > If the current task fails the check for the queried capability via
> > `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> > Issuing such denial messages unnecessarily can lead to a policy author
> > granting more privileges to a subject than needed to silence them.
> >
> > Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> > actually privileged.
> >
> > Signed-off-by: Christian Göttsche <cgzones at googlemail.com>
> > ---
> >  security/keys/keyctl.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> > index d54f73c558f7..19be69fa4d05 100644
> > --- a/security/keys/keyctl.c
> > +++ b/security/keys/keyctl.c
> > @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
> >         ret = -EACCES;
> >         down_write(&key->sem);
> >
> > -       if (!capable(CAP_SYS_ADMIN)) {
> > +       {
> > +               bool is_privileged_op = false;
> > +
> >                 /* only the sysadmin can chown a key to some other UID */
> >                 if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> > -                       goto error_put;
> > +                       is_privileged_op = true;
> >
> >                 /* only the sysadmin can set the key's GID to a group other
> >                  * than one of those that the current process subscribes to */
> >                 if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> > +                       is_privileged_op = true;
> > +
> > +               if (is_privileged_op && !capable(CAP_SYS_ADMIN))
> >                         goto error_put;
> >         }
>
> Hmm.  Using braces just to create a new scope is a bit hacky; I'll
> admit to using it to quickly create new local variables, but I only do
> so in debug/test situations, not production code.
>
> What if you move the CAP_SYS_ADMIN check down into the if-conditional
> where the code checks to see if CAP_SYS_ADMIN is needed when changing
> the UID?  It should be possible to structure the CAP_SYS_ADMIN check
> such that it is only executed if needed.  It's a little more
> complicated in the GID case, but I believe it should be doable.

This complication I exactly wanted to avoid.  For me the inner scope
encapsulates the all the logic around the capability check just fine
and is quite readable.  An alternative would be to create a new
function performing the checks and call it via

    if (!chown_key_capable(key, user, uid, group, gid))
        got error_put;

A minor inconvenience is the number of needed arguments (and the
actual code after inlining should be the same to the inner scope in
the end).

>
> --
> paul-moore.com