Re: 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@xxxxxxxxxx> wrote:
> > > On 04/17, Paul Moore wrote:
> > > > On Wed, Apr 17, 2019 at 10:57 AM Oleg Nesterov <oleg@xxxxxxxxxx> 
> > > > 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