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

From: Daniel Vetter <daniel.vetter@ffwll.ch>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jan Kara <jack@suse.cz>,
	andrew Morton <akpm@linux-foundation.org>,
	 Linux MM <linux-mm@kvack.org>,
	Hans Verkuil <hans.verkuil@cisco.com>,
	 Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
	Mel Gorman <mgorman@suse.de>,  stable <stable@vger.kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	 John Hubbard <jhubbard@nvidia.com>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	 LKML <linux-kernel@vger.kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	 Linux ARM <linux-arm-kernel@lists.infradead.org>,
	 linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	 "open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>
Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM
Date: Tue, 6 Oct 2020 13:56:53 +0200	[thread overview]
Message-ID: <CAKMK7uGrQq6tb2hMUSC-=JkTNMC2DvdQkcZncmVBKZ-0x6S61Q@mail.gmail.com> (raw)
In-Reply-To: <20201005175746.GA4734@nvidia.com>

On Mon, Oct 5, 2020 at 7:58 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> On Mon, Oct 05, 2020 at 07:53:08PM +0200, Jan Kara wrote:
> > On Mon 05-10-20 14:38:54, Jason Gunthorpe wrote:
> > > When get_vaddr_frames() does its hacky follow_pfn() loop it should never
> > > be allowed to extract a struct page from a normal VMA. This could allow a
> > > serious use-after-free problem on any kernel memory.
> > >
> > > Restrict this to only work on VMA's with one of VM_IO | VM_PFNMAP
> > > set. This limits the use-after-free problem to only IO memory, which while
> > > still serious, is an improvement.
> > >
> > > Cc: stable@vger.kernel.org
> > > Fixes: 8025e5ddf9c1 ("[media] mm: Provide new get_vaddr_frames() helper")
> > > Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> > >  mm/frame_vector.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> > > index 10f82d5643b6de..26cb20544b6c37 100644
> > > +++ b/mm/frame_vector.c
> > > @@ -99,6 +99,10 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> > >             if (ret >= nr_frames || start < vma->vm_end)
> > >                     break;
> > >             vma = find_vma_intersection(mm, start, start + 1);
> > > +           if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> > > +                   ret = -EINVAL;
> > > +                   goto out;
> > > +           }
> > >     } while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));
> >
> > Hum, I fail to see how this helps. If vma has no VM_IO or VM_PFNMAP flag,
> > we'd exit the loop (to out: label) anyway due to the loop termination
> > condition and why not return the frames we already have? Furthermore
> > find_vma_intersection() can return NULL which would oops in your check
> > then. What am I missing?
>
> Oh, nothing, you are right. It just didn't read naturally because
> hitting the wrong kind of VMA should be an error condition :\

Also follow_pfn checks for this same conditionat already too, so this
isn't really stopping anything bad from happening.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch