Re: [PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()

[Andrea Parri]

On Wed, May 29, 2019 at 07:38:46PM +0200, Jann Horn wrote:
> On Wed, May 29, 2019 at 6:21 PM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> > On 05/29, Jann Horn wrote:
> > > (I have no clue whatsoever what the relevant tree for this is, but I
> > > guess Oleg is the relevant maintainer?)
> >
> > we usually route ptrace changes via -mm tree, plus I lost my account on korg.
> >
> > > --- a/kernel/ptrace.c
> > > +++ b/kernel/ptrace.c
> > > @@ -324,6 +324,16 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> > >       return -EPERM;
> > >  ok:
> > >       rcu_read_unlock();
> > > +     /*
> > > +      * If a task drops privileges and becomes nondumpable (through a syscall
> > > +      * like setresuid()) while we are trying to access it, we must ensure
> > > +      * that the dumpability is read after the credentials; otherwise,
> > > +      * we may be able to attach to a task that we shouldn't be able to
> > > +      * attach to (as if the task had dropped privileges without becoming
> > > +      * nondumpable).
> > > +      * Pairs with a write barrier in commit_creds().
> > > +      */
> > > +     smp_rmb();
> >
> > (I am wondering if smp_acquire__after_ctrl_dep() could be used instead, just to
> >  make this code look more confusing)
> 
> Uuh, I had no idea that that barrier type exists. The helper isn't
> even explicitly mentioned in Documentation/memory-barriers.rst. I
> don't really want to use dark magic in the middle of ptrace access
> logic...
> 
> Anyway, looking at it, I think smp_acquire__after_ctrl_dep() doesn't
> make sense here; quoting the documentation: "A load-load control
> dependency requires a full read memory barrier, not simply a data
> dependency barrier to make it work correctly". IIUC
> smp_acquire__after_ctrl_dep() is for cases in which you would
> otherwise need a full memory barrier - smp_mb() - and you want to be
> able to reduce it to a read barrier.

It is supposed to be used when you want an ACQUIRE but you only have a
control dependency (so you "augment the dependency" with this barrier).

FWIW, I do agree on the "dark magic"..., and I'd strongly recommend to
not use this barrier (or, at least, to use it with high suspicion).

  Andrea