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Date:   Mon, 10 May 2021 10:50:31 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        Tomasz Figa <tfiga@chromium.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        linux-samsung-soc@vger.kernel.org
Subject: Re: [PULL] topic/iomem-mmap-vs-gup
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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.

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

Regards,
Jason