kernel BUG at kernel/cred.c:434!

[Casey Schaufler]

On 4/17/2019 9:27 AM, Oleg Nesterov 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?
>
> In fact, I think that something is already wrong if it is not called by
> user-space directly. Too late to ask, but why is this /proc/self/attr/
> magic not implemented via syscall(s) ?

Shell scripts, for one thing. It's a straightforward and appropriate
use of the /proc interface. System calls would require additional change
to existing programs, whereas using the /proc interface allows a good
deal to be done in the containing scripts.