Re: [PULL] topic/iomem-mmap-vs-gup

[Daniel Vetter]

On Mon, May 10, 2021 at 3:50 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Sat, May 08, 2021 at 09:46:41AM -0700, Linus Torvalds wrote:
>
> > I think follow_pfn() is ok for the actual "this is not a 'struct page'
> > backed area", and disabling that case is wrong even going forward.
>
> Every place we've audited using follow_pfn() has been shown to have
> some use-after-free bugs like Daniel describes, and a failure to check
> permissions bug too.
>
> All the other follow_pfn() users were moved to follow_pte() to fix the
> permissions check and this shifts the use-after-free bug away from
> being inside an MM API and into the caller mis-using the API by, say,
> extracting and using the PFN outside the pte lock.
>
> eg look at how VFIO wrongly uses follow_pte():
>
> static int follow_fault_pfn()
>         ret = follow_pte(vma->vm_mm, vaddr, &ptep, &ptl);
>                 *pfn = pte_pfn(*ptep);
>         pte_unmap_unlock(ptep, ptl);
>
>         // no protection that pte_pfn() is still valid!
>         use_pfn(*pfn)
>
> v4l is the only user that still has the missing permissions check
> security bug too - so there is no outcome that should keep
> follow_pfn() in the tree.
>
> At worst v4l should change to follow_pte() and use it wrongly like
> VFIO. At best we should delete all the v4l stuff.

yeah vfio is still broken for the case I care about. I think there's
also some questions open still about whether kvm really uses
mmu_notifier in all cases correctly, but iirc the one exception was
s390, which didn't have pci mmap and that's how it gets away with that
specific problem.

> Daniel I suppose we missed this relation to follow_pte(), so I agree
> that keeping a unsafe_follow_pfn() around is not good.

tbh I never really got the additional issue with the missing write
checks. That users of follow_pfn (or well follow_pte + immediate lock
dropping like vfio) don't subscribe to the pte updates in general is
the bug I'm seeing. That v4l also glosses over the read/write access
stuff is kinda just the icing on the cake :-) It's pretty well broken
even if it would check that.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch