Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default

Christoph Hellwig
2023-04-24T12:38:54Z

On Mon, Apr 24, 2023 at 09:28:07AM -0300, Jason Gunthorpe wrote:
> On Mon, Apr 24, 2023 at 11:17:55AM +0100, Lorenzo Stoakes wrote:
> > On Mon, Apr 24, 2023 at 02:43:56AM -0700, Christoph Hellwig wrote:
> > > I'm pretty sure DIRECT I/O reads that write into file backed mappings
> > > are out there in the wild.
>
> I wonder if that is really the case? I know people tried this with
> RDMA and it didn't get very far before testing uncovered data
> corruption and kernel crashes.. Maybe O_DIRECT has a much smaller race
> window so people can get away with it?

It absolutely causes all kinds of issues even with O_DIRECT. I'd be
all for trying to disallow it as it simplies a lot of things, but I fear
it's not going to stick.

> So, my suggestion was to mark the places where we want to allow this,
> eg O_DIRECT, and block everwhere else. Lorenzo, I would significantly
> par back the list you have.

I think an opt-in is a good idea no matter how many places end up
needing it. I'd prefer a less dramatic name and a better explanation
on why it should only be set when needed.

> I also suggest we force block it at some kernel lockdown level..
>
> Alternatively, perhaps we abuse FOLL_LONGTERM and prevent it from
> working with filebacked pages since, I think, the ease of triggering a
> bug goes up the longer the pages are pinned.

FOLL_LONGTERM on file backed pages is a nightmare. If you think you
can get away with prohibiting it for RDMA, and KVM doesn't need it
I'd be all for not allowing that at all.