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

Jason Gunthorpe
2023-04-24T12:28:15Z

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?

> I know Jason is keen on fixing this at a fundamental level and this flag is
> ultimately his suggestion, so it certainly doesn't stand in the way of this
> work moving forward.

Yeah, the point is to close it off, because while we wish it was
fixed properly, it isn't. We are still who knows how far away from it.

In the mean time this is a fairly simple way to oops the kernel,
especially with cases like io_uring and RDMA. So, I view it as a
security problem.

My general dislike was that io_uring protected itself from the
security problem and we left all the rest of the GUP users out to dry.

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 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.

Jason