Re: [PATCH v2] binder: fix use-after-free due to fdget() optimization

[Todd Kjos]

On Wed, Dec 5, 2018 at 4:40 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Dec 05, 2018 at 04:21:55PM -0800, Todd Kjos wrote:
>
> > > How about grabbing the references to all victims (*before* screwing with
> > > ksys_close()), sticking them into a structure with embedded callback_head
> > > and using task_work_add() on it, the callback doing those fput()?
> > >
> > > The callback would trigger before the return to userland, so observable
> > > timing of the final close wouldn't be changed.  And it would avoid the
> > > kludges like this.
> >
> > I'll rework it according to your suggestion. I had hoped to do this in a way
> > that doesn't require adding calls to non-exported functions since we are
> > trying to clean up binder (I hear you snickering) to be a better citizen and
> > not rely on internal functions that drivers shouldn't be using. I presume
> > there are no plans to export task_work_add()...
>
> Er...  Your variant critically depends upon binder being non-modular; if it
> *was* built as a module, you could
>         * lose the timeslice just after your fput()
>         * have another process hit the final fput() *and* close the struct file
>         * now that module refcount is not pinned by anything, get rmmod remove
> your module
>         * have the process in binder_ioctl() regain the timeslice and find the
> code under it gone.
>
> That's one of the reasons why such kludges are brittle as hell - normally you
> are guaranteed that once fdget() has succeeded, the final fput() won't happen
> until fdput().  With everything that guarantees in terms of code/data not going
> away under you.  This patch relies upon the lack of accesses to anything
> sensitive after that fput() added into binder_ioctl().  Which is actually
> true, but only because the driver is not modular...
>
> At least this variant (task_work_add()-based) doesn't depend on anything
> subtle - the lack of exports is the only problem there (IOW, it would've
> worked in a module if not for that).

Thanks for the detailed responses. I'll rework it for v3.