Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Sun, Nov 1, 2020 at 10:13 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 11/1/20 2:30 AM, Daniel Vetter wrote:
> > On Sun, Nov 1, 2020 at 6:22 AM John Hubbard <jhubbard@nvidia.com> wrote:
> >>
> >> On 10/31/20 7:45 AM, Daniel Vetter wrote:
> >>> On Sat, Oct 31, 2020 at 3:55 AM John Hubbard <jhubbard@nvidia.com> wrote:
> >>>> On 10/30/20 3:08 AM, Daniel Vetter wrote:
> >> ...
> >>>> By removing this check from this location, and changing from
> >>>> pin_user_pages_locked() to pin_user_pages_fast(), I *think* we end up
> >>>> losing the check entirely. Is that intended? If so it could use a comment
> >>>> somewhere to explain why.
> >>>
> >>> Yeah this wasn't intentional. I think I needed to drop the _locked
> >>> version to prep for FOLL_LONGTERM, and figured _fast is always better.
> >>> But I didn't realize that _fast doesn't have the vma checks, gup.c got
> >>> me a bit confused.
> >>
> >> Actually, I thought that the change to _fast was a very nice touch, btw.
> >>
> >>>
> >>> I'll remedy this in all the patches where this applies (because a
> >>> VM_IO | VM_PFNMAP can point at struct page backed memory, and that
> >>> exact use-case is what we want to stop with the unsafe_follow_pfn work
> >>> since it wreaks things like cma or security).
> >>>
> >>> Aside: I do wonder whether the lack for that check isn't a problem.
> >>> VM_IO | VM_PFNMAP generally means driver managed, which means the
> >>> driver isn't going to consult the page pin count or anything like that
> >>> (at least not necessarily) when revoking or moving that memory, since
> >>> we're assuming it's totally under driver control. So if pup_fast can
> >>> get into such a mapping, we might have a problem.
> >>> -Daniel
> >>>
> >>
> >> Yes. I don't know why that check is missing from the _fast path.
> >> Probably just an oversight, seeing as how it's in the slow path. Maybe
> >> the appropriate response here is to add a separate patch that adds the
> >> check.
> >>
> >> I wonder if I'm overlooking something, but it certainly seems correct to
> >> do that.
> >
> > You'll need the mmap_sem to get at the vma to be able to do this
> > check. If you add that to _fast, you made it as fast as the slow one.
>
> Arggh, yes of course. Strike that, please. :)
>
> > Plus there's _fast_only due to locking recurion issues in fast-paths
> > (I assume, I didn't check all the callers).
> >
> > I'm just wondering whether we have a bug somewhere with device
> > drivers. For CMA regions we always check in try_grab_page, but for dax
>
> OK, so here you're talking about a different bug than the VM_IO | VM_PFNMAP
> pages, I think. This is about the "FOLL_LONGTERM + CMA + gup/pup _fast"
> combination that is not allowed, right?

Yeah sorry, I got distracted reading code and noticed we might have
another issue.

> For that: try_grab_page() doesn't check anything, but try_grab_compound_head()
> does, but only for pup_fast, not gup_fast. That was added by commit
> df3a0a21b698d ("mm/gup: fix omission of check on FOLL_LONGTERM in gup fast
> path") in April.
>
> I recall that the patch was just plugging a very specific hole, as opposed
> to locking down the API against mistakes or confused callers. And it does
> seem that there are some holes.

Yup that's the one I've found.

> > I'm not seeing where the checks in the _fast fastpaths are, and that
> > all still leaves random device driver mappings behind which aren't
> > backed by CMA but still point to something with a struct page behind
> > it. I'm probably just missing something, but no idea what.
> > -Daniel
> >
>
> Certainly we've established that we can't check VMA flags by that time,
> so I'm not sure that there is much we can check by the time we get to
> gup/pup _fast. Seems like the device drivers have to avoid calling _fast
> with pages that live in VM_IO | VM_PFNMAP, by design, right? Or maybe
> you're talking about CMA checks only?

It's not device drivers, but everyone else. At least my understanding
is that VM_IO | VM_PFNMAP means "even if it happens to be backed by a
struct page, do not treat it like normal memory". And gup/pup_fast
happily break that. I tried to chase the history of that test, didn't
turn up anything I understood much:

commit 1ff8038988adecfde71d82c0597727fc239d4e8c
Author: Linus Torvalds <torvalds@g5.osdl.org>
Date:   Mon Dec 12 16:24:33 2005 -0800

   get_user_pages: don't try to follow PFNMAP pages

   Nick Piggin points out that a few drivers play games with VM_IO (why?
   who knows..) and thus a pfn-remapped area may not have that bit set even
   if remap_pfn_range() set it originally.

   So make it explicit in get_user_pages() that we don't follow VM_PFNMAP
   pages, since pretty much by definition they do not have a "struct page"
   associated with them.

   Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/mm/memory.c b/mm/memory.c
index 47c533eaa072..d22f78c8a381 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1009,7 +1009,7 @@ int get_user_pages(struct task_struct *tsk,
struct mm_struct *mm,
                       continue;
               }

-               if (!vma || (vma->vm_flags & VM_IO)
+               if (!vma || (vma->vm_flags & (VM_IO | VM_PFNMAP))
                               || !(vm_flags & vma->vm_flags))
                       return i ? : -EFAULT;


The VM_IO check is kinda lost in pre-history.

tbh I have no idea what the various variants of pup/gup are supposed
to be doing vs. these VMA flags in the various cases. Just smells a
bit like potential trouble due to randomly pinning stuff without the
owner of that memory having an idea what's going on.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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