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
