Re: [PATCH v2 4/5] LSM: Define SELinux function to measure security state

[Lakshmi Ramasubramanian]

On 7/16/20 11:54 AM, Stephen Smalley wrote:

> > The data for selinux-state in the above measurement is:
> > enabled=1;enforcing=0;checkreqprot=1;network_peer_controls=1;open_perms=1;extended_socket_class=1;always_check_network=0;cgroup_seclabel=1;nnp_nosuid_transition=1;genfs_seclabel_symlinks=0;
> >
> > The data for selinux-policy-hash in the above measurement is
> > the SHA256 hash of the SELinux policy.
>
> Can you show an example of how to verify that the above measurement
> matches a given state and policy, e.g. the sha256sum commands and
> inputs to reproduce the same from an expected state and policy?
Sure - I'll provide an example.

> > +/* Pre-allocated buffer used for measuring state */
> > +static char *selinux_state_string;
> > +static size_t selinux_state_string_len;
> > +static char *selinux_state_string_fmt =
> > +       "%s=%d;%s=%d;%s=%d;%s=%d;%s=%d;%s=%d;%s=%d;%s=%d;%s=%d;%s=%d;";
> > +
> > +void __init selinux_init_measurement(void)
> > +{
> > +       selinux_state_string_len =
> > +       snprintf(NULL, 0, selinux_state_string_fmt,
> > +       "enabled", 0,
> > +       "enforcing", 0,
> > +       "checkreqprot", 0,
> > +       selinux_policycap_names[POLICYDB_CAPABILITY_NETPEER], 0,
> > +       selinux_policycap_names[POLICYDB_CAPABILITY_OPENPERM], 0,
> > +       selinux_policycap_names[POLICYDB_CAPABILITY_EXTSOCKCLASS], 0,
> > +       selinux_policycap_names[POLICYDB_CAPABILITY_ALWAYSNETWORK], 0,
> > +       selinux_policycap_names[POLICYDB_CAPABILITY_CGROUPSECLABEL], 0,
> > +       selinux_policycap_names[POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION], 0,
> > +       selinux_policycap_names[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS],
> > +       0);
>
> I was thinking you'd dynamically construct the format string with a
> for loop from 0 to POLICYDB_CAPABILITY_MAX
> and likewise for the values so that we wouldn't have to patch this
> code every time we add a new one.
That's a good point - will do.

> > +
> > +       if (selinux_state_string_len < 0)
> > +               return;
>
> How can this happen legitimately (i.e. as a result of something other
> than a kernel bug)?
Since snprintf can return an error I wanted to handle that. But I agree 
this should not happen for the input data to snprintf used here.

> > +
> > +       ++selinux_state_string_len;
> > +
> > +       selinux_state_string = kzalloc(selinux_state_string_len, GFP_KERNEL);
> > +       if (!selinux_state_string)
> > +               selinux_state_string_len = 0;
> > +}
>
> Not sure about this error handling approach (silent, proceeding as if
> the length was zero and then later failing with ENOMEM on every
> attempt?). I'd be more inclined to panic/BUG here but I know Linus
> doesn't like that.
I am not sure if failing (kernel panic/BUG) to "measure" LSM data under 
memory pressure conditions is the right thing. But I am open to treating 
this error as a fatal error. Please let me know.

> > +       if (ret)
> > +               pr_err("%s: error %d\n", __func__, ret);
>
> This doesn't seem terribly useful as an error message; I'd be inclined
> to drop it.
Will do.

thanks,
 -lakshmi