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

On Sat, Mar 21, 2015 at 04:18:32PM +0200, Konstantin Belousov wrote:
> 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);                                        
> -                                                                               

fork1 does:

        proc_set_cred(newproc, p1->p_ucred);

p1 is unlocked, so whatever memory p1->p_ucred points to may already be
freed.

        /*
         * Initialize resource accounting for the child process.
         */
        error = racct_proc_fork(p1, newproc);
        if (error != 0) {
                error = EAGAIN;
                goto fail1;
        }

racct_proc_fork -> racct_add_locked results in accessing such
now-possibly-freed credentials.

do_fork which properly assigns credentials (from a stable source
(td_ucred) + grabs a reference) is called later.

The patch in question moves aforementioned assignent earlier to replace
unsafe one with p1->p_ucred.

-- 
Mateusz Guzik <mjguzik gmail.com>