Re: GUP guarantees wrt to userspace mappings redesign

From: Oleg Nesterov
Date: Mon May 02 2016 - 14:43:23 EST

On 05/02, Kirill A. Shutemov wrote:
> On Mon, May 02, 2016 at 06:22:11PM +0200, Oleg Nesterov wrote:

> > > So we have pages pinned by a driver and the driver expects the pinned
> > > pages to be mapped into userspace, then __replace_page() kicks in and put
> > > different page there -- driver's expectation is broken.
> >
> > Yes... but I don't understand the problem space. I mean, I do not know why
> > this driver should expect this, how it can be broken, etc.
> >
> > I do not even understand why "initiated by other process" can make any
> > difference... Unless this driver somehow controls all threads which could
> > have this page mapped.
> Okay, my understanding is following:
> Some drivers (i.e. vfio) rely on get_user_page{,_fast}() to pin the memory
> and expect pinned pages to be mapped into userspace until the pin is gone.
> This memory is used to communicate between kernel and userspace.

Thanks Kirill.

Then I think uprobes should be fine,

> I don't think there's something to fix on uprobe side. It's part of
> debugging interface. Debuggers can be destructive, nothing new there.

Yes, exactly. And as for uprobes in particular, __replace_page() can
only be called of vma->vm_file and and the mapping is private/executable,
VM_MAYSHARE must not be set.

Unlikely userspace can read or write to this memory to communicate with
kernel or something else.