Re: kernel BUG at kernel/cred.c:434!

[Paul Moore]

On Thu, Apr 18, 2019 at 10:42 PM Yang Yingliang
<yangyingliang@xxxxxxxxxx> wrote:
> On 2019/4/19 10:04, Paul Moore wrote:
> > On Wed, Apr 17, 2019 at 10:50 PM Yang Yingliang
> > <yangyingliang@xxxxxxxxxx> wrote:
> >> On 2019/4/18 8:24, Casey Schaufler wrote:
> >>> On 4/17/2019 4:39 PM, Paul Moore wrote:
> >>>> 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
> > I'm sorry, you've lost me.  After override_creds() returns
> > current->cred and current->real_cred are not going to be the same,
> > yes?
>
> It's possible the new  cred is equal to current->real_cred and
> current->cred,
> so after overrides_creds(), they have the same value.

Both task_struct.cred and task_struct.real_cred are pointer values,
assuming that one uses prepare_creds() to allocate/initialize a new
cred struct for use with override_creds() then the newly created cred
should never be equal to task_struct.real_cred.  Am I missing
something, or are you thinking of something else?

-- 
paul moore
www.paul-moore.com