Re: [PATCH] cred: Use RCU primitives to access RCU pointers

[Jann Horn]

On Tue, Jan 28, 2020 at 6:04 PM Amol Grover <frextrite@xxxxxxxxx> wrote:
> On Tue, Jan 28, 2020 at 10:30:19AM +0100, Jann Horn wrote:
> > On Tue, Jan 28, 2020 at 8:28 AM Amol Grover <frextrite@xxxxxxxxx> wrote:
> > > task_struct.cred and task_struct.real_cred are annotated by __rcu,
> >
> > task_struct.cred doesn't actually have RCU semantics though, see
> > commit d7852fbd0f0423937fa287a598bfde188bb68c22. For task_struct.cred,
> > it would probably be more correct to remove the __rcu annotation?
> >
>
> Hi Jann,
>
> I went through the commit you mentioned. If I understand it correctly,
> ->cred was not being accessed concurrently (via RCU), hence, a non_rcu
> flag was introduced, which determined if the clean-up should wait for
> RCU grace-periods or not. And since, the changes were 'thread local'
> there was no need to wait for an entire RCU GP to elapse.

Yeah.

> The commit too, as you said, mentions the removal of __rcu annotation.
> However, simply removing the annotation won't work, as there are quite a
> few instances where RCU primitives are used. Even get_current_cred()
> uses RCU APIs to get a reference to ->cred.

Luckily, there aren't too many places that directly access ->cred,
since luckily there are helper functions like get_current_cred() that
will do it for you. Grepping through the kernel, I see:

Places that need adjustment:

include/linux/cred.h:   rcu_dereference_protected(current->cred, 1)
kernel/auditsc.c: * the only situations where tsk->cred may be
accessed without an rcu read lock.
kernel/auditsc.c:       cred = rcu_dereference_check(tsk->cred, tsk ==
current || task_creation);
kernel/cred.c:  rcu_assign_pointer(task->cred, new);
kernel/cred.c:  rcu_assign_pointer(current->cred, new);
kernel/cred.c:  rcu_assign_pointer(current->cred, old);


Places that already don't use RCU accessors:

drivers/virt/vboxguest/vboxguest_linux.c:       if
(from_kuid(current_user_ns(), current->cred->uid) == 0)
kernel/cred.c:  BUG_ON(cred == current->cred);
kernel/cred.c:  kdebug("exit_creds(%u,%p,%p,{%d,%d})", tsk->pid,
tsk->real_cred, tsk->cred,
kernel/cred.c:         atomic_read(&tsk->cred->usage),
kernel/cred.c:         read_cred_subscribers(tsk->cred));
kernel/cred.c:  cred = (struct cred *) tsk->cred;
kernel/cred.c:  tsk->cred = NULL;
kernel/cred.c:  old = task->cred;
kernel/cred.c:          !p->cred->thread_keyring &&
kernel/cred.c:          p->real_cred = get_cred(p->cred);
kernel/cred.c:          get_cred(p->cred);
kernel/cred.c:          alter_cred_subscribers(p->cred, 2);
kernel/cred.c:                 p->cred, atomic_read(&p->cred->usage),
kernel/cred.c:                 read_cred_subscribers(p->cred));
kernel/cred.c:          atomic_inc(&p->cred->user->processes);
kernel/cred.c:  p->cred = p->real_cred = get_cred(new);
kernel/cred.c:  BUG_ON(task->cred != old);
kernel/cred.c:  const struct cred *old = current->cred;
kernel/cred.c:   * '->cred' pointer, not the '->real_cred' pointer that is
kernel/cred.c:  const struct cred *override = current->cred;
kernel/cred.c:         cred == tsk->cred ? "[eff]" : "");
kernel/cred.c:  if (tsk->cred == tsk->real_cred) {
kernel/cred.c:          if (unlikely(read_cred_subscribers(tsk->cred) < 2 ||
kernel/cred.c:                       creds_are_invalid(tsk->cred)))
kernel/cred.c:                       read_cred_subscribers(tsk->cred) < 1 ||
kernel/cred.c:                       creds_are_invalid(tsk->cred)))
kernel/cred.c:  if (tsk->cred != tsk->real_cred)
kernel/cred.c:          dump_invalid_creds(tsk->cred, "Effective", tsk);
kernel/cred.c:         tsk->real_cred, tsk->cred,
kernel/cred.c:         atomic_read(&tsk->cred->usage),
kernel/cred.c:         read_cred_subscribers(tsk->cred));
kernel/fork.c:  atomic_dec(&p->cred->user->processes);
security/security.c:    lsm_early_cred((struct cred *) current->cred);
security/smack/smack_lsm.c:     struct cred *cred = (struct cred *)
current->cred;
security/tomoyo/common.c:           (!uid_eq(task->cred->uid,
GLOBAL_ROOT_UID) ||
security/tomoyo/common.c:            !uid_eq(task->cred->euid,
GLOBAL_ROOT_UID)))


Places that don't use RCU and are broken:

security/smack/smack_lsm.c:     struct smack_known *tkp =
smk_of_task(smack_cred(tsk->cred));

So actually, the number of places that already don't use RCU accessors
is much higher than the number of places that use them.

> So, currently, maybe we
> should continue to use RCU APIs and leave the __rcu annotation in?
> (Until someone who takes it on himself to remove __rcu annotation and
> fix all the instances). Does that sound good? Or do you want me to
> remove __rcu annotation and get the process started?

I don't think it's a good idea to add more uses of RCU APIs for
->cred; you shouldn't "fix" warnings by making the code more wrong.

If you want to fix this, I think it would be relatively easy to fix
this properly - as far as I can tell, there are only seven places that
you'll have to change, although you may have to split it up into three
patches.