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

[Lorenzo Stoakes]

On Mon, Apr 24, 2023 at 03:54:48PM -0300, Jason Gunthorpe wrote:
> On Mon, Apr 24, 2023 at 07:22:03PM +0100, Lorenzo Stoakes wrote:
>
> > OK I guess you mean the folio lock :) Well there is
> > unpin_user_pages_dirty_lock() and unpin_user_page_range_dirty_lock() and
> > also set_page_dirty_lock() (used by __access_remote_vm()) which should
> > avoid this.
>
> It has been a while, but IIRC, these are all basically racy, the
> comment in front of set_page_dirty_lock() even says it is racy..
>
> The race is that a FS cleans a page and thinks it cannot become dirty,
> and then it becomes dirty - and all variations of that..
>
> Looking around a bit, I suppose what I'd expect to see is a sequence
> sort of like what do_page_mkwrite() does:
>
>         /* Synchronize with the FS and get the page locked */
>      	ret = vmf->vma->vm_ops->page_mkwrite(vmf);
> 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
> 		return ret;
> 	if (unlikely(!(ret & VM_FAULT_LOCKED))) {
> 		lock_page(page);
> 		if (!page->mapping) {
> 			unlock_page(page);
> 			return 0; /* retry */
> 		}
> 		ret |= VM_FAULT_LOCKED;
> 	} else
> 		VM_BUG_ON_PAGE(!PageLocked(page), page);
>
> 	/* Write to the page with the CPU */
> 	va = kmap_local_atomic(page);
> 	memcpy(va, ....);
> 	kunmap_local_atomic(page);
>
> 	/* Tell the FS and unlock it. */
> 	set_page_dirty(page);
> 	unlock_page(page);
>
> I don't know if this is is exactly right, but it seems closerish
>
> So maybe some kind of GUP interfaces that returns single locked pages
> is the right direction? IDK
>
> Or maybe we just need to make a memcpy primitive that works while
> holding the PTLs?
>

I think this patch suggestion has scope crept from 'incremental
improvement' to 'major rework of GUP' at this point. Also surely you'd want
to obtain the PTL of all mappings to a file? This seems really unworkable
and I don't think holding a folio lock over a long period is sensible
either.

> > We definitely need to keep ptrace and /proc/$pid/mem functioning correctly,
> > and I given the privilege levels required I don't think there's a security
> > issue there?
>
> Even root is not allowed to trigger data corruption or oops inside the
> kernel.
>
> Jason

Of course, but isn't this supposed to be an incremental fix? It feels a
little contradictory to want to introduce a flag intentionally to try to
highlight brokenness then to not accept any solution that doesn't solve
that brokenness.

In any case, I feel that this patch isn't going to go anywhere as-is, it's
insufficiently large to solve the problem as a whole (I think that's a
bigger problem we can return to later), and there appears to be no taste
for an incremental improvement, even from the suggester :)

As a result, I suggest we take the cautious route in order to unstick the
vmas patch series - introduce an OPT-IN flag which allows the check to be
made, and update io_uring to use this.

That way it defers the larger discussion around this improvement, avoids
breaking anything, provides some basis in code for this check and is a net,
incremental small and digestible improvement.