Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

From: Arnd Bergmann
Date: Sun Sep 20 2020 - 09:56:17 EST

On Sun, Sep 20, 2020 at 12:09 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Sep 18, 2020 at 05:16:15PM +0200, Christoph Hellwig wrote:
> > On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote:
> > > Said that, why not provide a variant that would take an explicit
> > > "is it compat" argument and use it there? And have the normal
> > > one pass in_compat_syscall() to that...
> >
> > That would help to not introduce a regression with this series yes.
> > But it wouldn't fix existing bugs when io_uring is used to access
> > read or write methods that use in_compat_syscall(). One example that
> > I recently ran into is drivers/scsi/sg.c.
> So screw such read/write methods - don't use them with io_uring.
> That, BTW, is one of the reasons I'm sceptical about burying the
> decisions deep into the callchain - we don't _want_ different
> data layouts on read/write depending upon the 32bit vs. 64bit
> caller, let alone the pointer-chasing garbage that is /dev/sg.

Would it be too late to limit what kind of file descriptors we allow
io_uring to read/write on?

If io_uring can get changed to return -EINVAL on trying to
read/write something other than S_IFREG file descriptors,
that particular problem space gets a lot simpler, but this
is of course only possible if nobody actually relies on it yet.