kernel BUG at kernel/cred.c:434!

[Yang Yingliang]

Hi, Casey

On 2019/4/18 8:24, Casey Schaufler wrote:
> On 4/17/2019 4:39 PM, Paul Moore wrote:
>> On Wed, Apr 17, 2019 at 12:27 PM Oleg Nesterov <oleg at redhat.com> wrote:
>>> On 04/17, Paul Moore wrote:
>>>> On Wed, Apr 17, 2019 at 10:57 AM Oleg Nesterov <oleg at redhat.com> 
>>>> wrote:
>>>>> On 04/17, Paul Moore wrote:
>>>>>> I'm tempted to simply return an error in selinux_setprocattr() if
>>>>>> the task's credentials are not the same as its real_cred;
>>>>> What about other modules? I have no idea what smack_setprocattr() is,
>>>>> but it too does prepare_creds/commit creds.
>>>>>
>>>>> it seems that the simplest workaround should simply add the 
>>>>> additional
>>>>> cred == real_cred into proc_pid_attr_write().
>>>> Yes, that is simple, but I worry about what other LSMs might want to
>>>> do.  While I believe failing if the effective creds are not the same
>>>> as the real_creds is okay for SELinux (possibly Smack too), I worry
>>>> about what other LSMs may want to do.  After all,
>>>> proc_pid_attr_write() doesn't change the the creds itself, that is
>>>> something the specific LSMs do.
>>> Yes, but if proc_pid_attr_write() is called with cred != real_cred then
>>> something is already wrong?
>> True, or at least I would think so.
>>
>> Looking at the current tree there are three LSMs which implement
>> setprocattr hooks: SELinux, Smack, and AppArmor.  I know Casey has
>> already mentioned that he wasn't able to trigger the problem in Smack,
>> but looking at smack_setprocattr() I see the similar commit_creds()
>> usage so I would expect the same problem in Smack; what say you Casey?
>
> I say that my test program runs without ill effect. I call acct()
> with "/proc/self/attr/current", which succeeds and enables accounting
> just like it is supposed to. I then have the program open
> "/proc/self/attr/current" and read it, all of which goes swimmingly.
> When Smack frees a cred it usually does not free any memory of its
> own, so it is conceivable that I'm just getting lucky. Or, I may not
> have sufficient debug enabled.
>
>>   Looking at apparmor_setprocattr(), it appears that it too could end
>> up calling commit_creds() via aa_set_current_hat().
>>
>> Since it looks like all three LSMs which implement the setprocattr
>> hook are vulnerable I'm open to the idea that proc_pid_attr_write() is
>> a better choice for the cred != read_cred check, but I would want to
>> make sure John and Casey are okay with that.
>>
>> John?
>>
>> Casey?
>
> I'm fine with the change going into proc_pid_attr_write().
The cred != real_cred checking is not enough.

Consider this situation, when doing override, cred, real_cred and 
new_cred are all same:

after override_creds()    cred == real_cred == new1_cred
after prepare_creds()     new2_cred
after commit_creds()     becasue the check is false, so cred == 
real_cred == new2_cred
after revert_creds()        cred == new1_cred, real_cred == new2_cred

It will cause cred != real_cred finally.


Regards,
Yang