On 10/28/2013 03:00 PM, Stephen Smalley wrote:
> On 10/28/2013 02:24 PM, Daniel J Walsh wrote:
>> On 10/28/2013 02:10 PM, Paul Moore wrote:
>>> On Monday, October 28, 2013 12:58:55 PM Stephen Smalley wrote:
>>>> On 10/28/2013 11:56 AM, Eric Paris wrote:
>>>>> On Mon, 2013-10-28 at 10:46 -0400, Daniel J Walsh wrote:
>>>>>> Maybe the solution here is to add logging messages to the function.
>>>>>>
>>>>>> My opionion is that if something is wrong with SELinux, IE The labels
>>>>>> are wrong, the policy is wrong or the app is wrong, we should not
>>>>>> block in permissive mode.
>>>>>>
>>>>>> Having the tool write "foobar_t is not a valid source context" would
>>>>>> be better then what we have now, which is a silent denial even in
>>>>>> permissive mode.>
>>>>> I understand Stephen's argument. But agree with dwalsh/bigon that
>>>>> hiding this in the library is a lot better than moving the logic to
>>>>> userspace programs. So this might not be so super simple to do. How
>>>>> about the idea of a new interface which always returns 0 in
>>>>> permissive? But it does a couple of extra things. These are just rough
>>>>> early thoughts....
>>>>>
>>>>> 0) new interface just like avc_has_perm() but which always returns 0
>>>>> in permissive.
>>>>>
>>>>> 1) a new SELINX_USER_ERR audit message. On EINVAL we check if the
>>>>> scontext/tcontext are valid and print the equivalent of a SELINUX_ERR
>>>>> message into the audit log if not.
>>>>>
>>>>> 2) a new /sys/fs/selinux/context like mechanism, which will both
>>>>> validate the context and will force it into the sid cache. So
>>>>> subsequent broken calls to avc_has_perm() will not generate a second
>>>>> SELINX_USER_ERR message, since the second call to 'access' will find a
>>>>> valid type and will give a denial for that unlabeled_t type?
>>>>>
>>>>> maybe /sys/fs/selinux/access should be changed/new interface added to
>>>>> do all of this in kernel? generating a real SELINUX_ERR in kernel and
>>>>> forcing the invalid label into the sid cache?
>>>>>
>>>>> I really do think that userspace object managers should be allowed to
>>>>> call avc_has_perm() and either get an error that should be handled as
>>>>> a hard failure or a 0... checking permissive in userspace object
>>>>> managers just seems prone to breakage...
>>>>
>>>> I'm ok with changing avc_has_perm as long as: a) Something gets
>>>> logged/audited so you'll see that something went wrong in permissive mode
>>>> and not just get silent failures in enforcing mode,
>>>>
>>>> b) We are careful about what error conditions are remapped to 0 in
>>>> permissive mode. If we just hit a memory allocation failure, we
>>>> shouldn't hide that from the caller. It should only affect things
>>>> relating to policy.
>>
>>> I'm far from an expert on the SELinux userland libraries, but as far as my
>>> two cents are concerned I like the idea of changing avc_has_perm() to
>>> incorporate the permissive/enforcing logic. I think asking applications to
>>> worry about things like that is a step in the wrong direction.
>>
>>> I also agree with Stephen's comments above: logging is important and the
>>> only failures that should be ignored by avc_has_perm() are those relating
>>> to access denials from policy, error conditions should propagate to the
>>> caller.
>>
>> The only functions that can fail are the following:
>>
>> rc = avc_lookup(ssid, tsid, tclass, requested, aeref);
>> if (rc) {
>> rc = security_compute_av_flags_raw(ssid->ctx, tsid->ctx,
>> tclass, requested,
>> &entry.avd);
>> if (rc)
>> goto out;
>> rc = avc_insert(ssid, tsid, tclass, &entry, aeref);
>>
>>
>> Are we stating that we should check for EINVAL and return 0 if permissive, or
>> is there some more complicated thing we should look up.
>>
>> I forget the original bugzilla that caused us to do this check but it was
>> something to do with dbus running witht he wrong context, I believe.
>
> I think the only error case of interest is when
> security_compute_av_flags_raw() returns -1 with errno EINVAL. That can
> mean that the source or target security context are no longer valid,
> e.g. due to a policy reload since the time the SIDs were allocated, or
> that the target security class is invalid, e.g. a userspace security
> class unknown to the loaded policy. (Or it could just represent random
> memory corruption in the application, of course).
>
> Returning 0 from avc_has_perm_noaudit() could be a source of subtle bugs
> without ensuring that the avd has been initialized sanely.
> avc_has_perm() initializes it before calling avc_has_perm_noaudit(), but
> maybe that should be moved inside of avc_has_perm_noaudit() if avd is set?
>
> Then the subsequent avc_audit() call by avc_has_perm() should set denied
> to 0 (as avd->allowed will be initialized to 0), but audited will be set
> to 0 because avd->auditdeny will be initialized to 0 too. So we'll
> audit nothing and get no indication of the error at that point.
>
> We could instead initialize avd in a similar way as the kernel does:
> static void avd_init(struct av_decision *avd)
> {
> avd->allowed = 0;
> avd->auditallow = 0;
> avd->auditdeny = 0xffffffff;
> avd->seqno = avc_cache.latest_notif;
> avd->flags = 0;
> }
>
> And call that on entry to avc_has_perm_noaudit() if avd is non-NULL.
> Then we'll get an audit message as well, although the security contexts
> may be invalid (but will be shown as their string form) or the class may
> be invalid (in which case we'll get a null string).
Like this:
diff --git a/libselinux/src/avc.c b/libselinux/src/avc.c
index 802a07f..f14eeb7 100644
--- a/libselinux/src/avc.c
+++ b/libselinux/src/avc.c
@@ -739,6 +739,16 @@ void avc_audit(security_id_t ssid, security_id_t tsid,
hidden_def(avc_audit)
+
+static void avd_init(struct av_decision *avd)
+{
+ avd->allowed = 0;
+ avd->auditallow = 0;
+ avd->auditdeny = 0xffffffff;
+ avd->seqno = avc_cache.latest_notif;
+ avd->flags = 0;
+}
+
int avc_has_perm_noaudit(security_id_t ssid,
security_id_t tsid,
security_class_t tclass,
@@ -751,6 +761,9 @@ int avc_has_perm_noaudit(security_id_t ssid,
access_vector_t denied;
struct avc_entry_ref ref;
+ if (avd)
+ avd_init(avd);
+
if (!avc_using_threads && !avc_app_main_loop) {
(void)avc_netlink_check_nb();
}
@@ -783,6 +796,10 @@ int avc_has_perm_noaudit(security_id_t ssid,
rc = security_compute_av_flags_raw(ssid->ctx, tsid->ctx,
tclass, requested,
&entry.avd);
+ if (rc && errno == EINVAL && !avc_enforcing) {
+ rc = errno = 0;
+ goto out;
+ }
if (rc)
goto out;
rc = avc_insert(ssid, tsid, tclass, &entry, aeref);
@@ -821,8 +838,6 @@ int avc_has_perm(security_id_t ssid, security_id_t tsid,
struct av_decision avd;
int errsave, rc;
- memset(&avd, 0, sizeof(avd));
-
rc = avc_has_perm_noaudit(ssid, tsid, tclass, requested, aeref, &avd);
errsave = errno;
avc_audit(ssid, tsid, tclass, requested, &avd, rc, auditdata);
