Re: [PATCH 1/3] fork: assign refed credentials earlier

On Sat, Mar 21, 2015 at 02:57:22AM +0100, Mateusz Guzik wrote:
> On Sat, Mar 21, 2015 at 03:51:51AM +0200, Konstantin Belousov wrote:
> > On Sat, Mar 21, 2015 at 02:00:38AM +0100, Mateusz Guzik wrote:
> > > From: Mateusz Guzik <mjg_at_freebsd.org>
> > > 
> > > Prior to this change the kernel would take p1's credentials and assign
> > > them tempororarily to p2. But p1 could change credentials at that time
> > > and in effect give us a use-after-free.
> > In which way could it change the credentials ?  The assigned credentials
> > are taken from td_ucred, which, I thought, are guaranteed to be stable
> > for the duration of a syscall.
> > 
> 
> It takes thread's credential in do_fork. But initial copy is taken
> unlocked from struct proc.
> 
> Relevant part of the diff:
> > > _at__at_ -870,7 +867,7 _at__at_ fork1(struct thread *td, int flags, int pages, struct proc **procp,
> > >  	 * XXX: This is ugly; when we copy resource usage, we need to bump
> > >  	 *      per-cred resource counters.
> > >  	 */
> > > -	proc_set_cred(newproc, p1->p_ucred);
> > > +	proc_set_cred(newproc, crhold(td->td_ucred));
> > >  

I do not understand your note, nor I see the chunk above in the patches
you send.  Below is the citation from the patch 1:

_at__at_ -410,9 +410,6 _at__at_ do_fork(struct thread *td, int flags, struct proc *p2,      
+struct thread *td2,                                                            
        bzero(&p2->p_startzero,                                                 
            __rangeof(struct proc, p_startzero, p_endzero));                    
                                                                                
-       crhold(td->td_ucred);                                                   
-       proc_set_cred(p2, td->td_ucred);                                        
-