Date: Sat, 21 Mar 2015 16:18:32 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Mateusz Guzik <mjguzik@gmail.com>, jenkins-admin@freebsd.org, freebsd-current@freebsd.org, Mateusz Guzik <mjg@freebsd.org> Subject: Re: [PATCH 1/3] fork: assign refed credentials earlier Message-ID: <20150321141832.GH2379@kib.kiev.ua> In-Reply-To: <20150321015722.GC27736@dft-labs.eu> References: <20150320122125.GP2379@kib.kiev.ua> <1426899640-6599-1-git-send-email-mjguzik@gmail.com> <1426899640-6599-2-git-send-email-mjguzik@gmail.com> <20150321015151.GF2379@kib.kiev.ua> <20150321015722.GC27736@dft-labs.eu>
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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@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:
> > > @@ -870,7 +867,7 @@ 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:
@@ -410,9 +410,6 @@ 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);
-
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