Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

[Jason Gunthorpe]

On Fri, Oct 02, 2020 at 07:53:03PM +0200, Daniel Vetter wrote:
> For $reasons I've stumbled over this code and I'm not sure the change
> to the new gup functions in 55a650c35fea ("mm/gup: frame_vector:
> convert get_user_pages() --> pin_user_pages()") was entirely correct.
> 
> This here is used for long term buffers (not just quick I/O) like
> RDMA, and John notes this in his patch. But I thought the rule for
> these is that they need to add FOLL_LONGTERM, which John's patch
> didn't do.
> 
> There is already a dax specific check (added in b7f0554a56f2 ("mm:
> fail get_vaddr_frames() for filesystem-dax mappings")), so this seems
> like the prudent thing to do.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-samsung-soc@xxxxxxxxxxxxxxx
> Cc: linux-media@xxxxxxxxxxxxxxx
> Hi all,
> 
> I stumbled over this and figured typing this patch can't hurt. Really
> just to maybe learn a few things about how gup/pup is supposed to be
> used (we have a bit of that in drivers/gpu), this here isn't really
> ralated to anything I'm doing.

FOLL_FORCE is a pretty big clue it should be FOLL_LONGTERM, IMHO

> I'm also wondering whether the explicit dax check should be removed,
> since FOLL_LONGTERM should take care of that already.

Yep! Confirms the above!

This get_vaddr_frames() thing looks impossible to use properly. How on
earth does a driver guarentee

 "If @start belongs to VM_IO | VM_PFNMAP vma, we don't touch page
 structures and the caller must make sure pfns aren't reused for
 anything else while he is using them."

The only possible way to do that is if the driver restricts the VMAs
to ones it owns and interacts with the vm_private data to refcount
something.

Since every driver does this wrong anything that uses this is creating
terrifying security issues.

IMHO this whole API should be deleted :(

Jason