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. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
