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From:   Souptick Joarder <jrdr.linux@gmail.com>
Date:   Wed, 19 Dec 2018 17:16:09 +0530
Message-ID: <CAFqt6zbLrw5HENBiLXxPFqo7kk8uBJd3z-+C9Fnkej7u3W2i1w@mail.gmail.com>
Subject: Re: [PATCH v4 4/9] drm/rockchip/rockchip_drm_gem.c: Convert to use vm_insert_range
To:     Russell King - ARM Linux <linux@armlinux.org.uk>
Cc:     Michal Hocko <mhocko@suse.com>, Heiko Stuebner <heiko@sntech.de>,
        Linux-MM <linux-mm@kvack.org>, airlied@linux.ie,
        hjc@rock-chips.com, Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-rockchip@lists.infradead.org,
        dri-devel@lists.freedesktop.org,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-arm-kernel@lists.infradead.org
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On Wed, Dec 19, 2018 at 3:02 PM Russell King - ARM Linux
<linux@armlinux.org.uk> wrote:
>
> On Wed, Dec 19, 2018 at 09:01:09AM +0530, Souptick Joarder wrote:
> > On Tue, Dec 18, 2018 at 6:31 PM Russell King - ARM Linux
> > <linux@armlinux.org.uk> wrote:
> > >
> > > On Tue, Dec 18, 2018 at 06:24:29PM +0530, Souptick Joarder wrote:
> > > > On Tue, Dec 18, 2018 at 6:03 PM Russell King - ARM Linux
> > > > <linux@armlinux.org.uk> wrote:
> > > > >
> > > > > On Tue, Dec 18, 2018 at 05:36:04PM +0530, Souptick Joarder wrote:
> > > > > > On Tue, Dec 18, 2018 at 3:27 PM Russell King - ARM Linux
> > > > > > <linux@armlinux.org.uk> wrote:
> > > > > > > This looks like a change in behaviour.
> > > > > > >
> > > > > > > If user_count is zero, and offset is zero, then we pass into
> > > > > > > vm_insert_range() a page_count of zero, and vm_insert_range() does
> > > > > > > nothing and returns zero.
> > > > > > >
> > > > > > > However, as we can see from the above code, the original behaviour
> > > > > > > was to return -ENXIO in that case.
> > > > > >
> > > > > > I think these checks are not necessary. I am not sure if we get into mmap
> > > > > > handlers of driver with user_count = 0.
> > > > >
> > > > > I'm not sure either, I'm just pointing out the change of behaviour.
> > > >
> > > > Ok. I think feedback from Heiko might be helpful here :)
> > > >
> > > > >
> > > > > > > The other thing that I'm wondering is that if (eg) count is 8 (the
> > > > > > > object is 8 pages), offset is 2, and the user requests mapping 6
> > > > > > > pages (user_count = 6), then we call vm_insert_range() with a
> > > > > > > pages of rk_obj->pages + 2, and a pages_count of 6 - 2 = 4. So we
> > > > > > > end up inserting four pages.
> > > > > >
> > > > > > Considering the scenario, user_count will remain 8 (user_count =
> > > > > > vma_pages(vma) ). ? No ?
> > > > > > Then we call vm_insert_range() with a pages of rk_obj->pages + 2, and
> > > > > > a pages_count
> > > > > > of 8 - 2 = 6. So we end up inserting 6 pages.
> > > > > >
> > > > > > Please correct me if I am wrong.
> > > > >
> > > > > vma_pages(vma) is the number of pages that the user requested, it is
> > > > > the difference between vma->vm_end and vma->vm_start in pages.  As I
> > > > > said above, "the user requests mapping 6 pages", so vma_pages() will
> > > > > be 6, and so user_count will also be 6.  You are passing
> > > > > user_count - offset into vm_insert_range(), which will be 6 - 2 = 4
> > > > > in my example.  This is two pages short of what the user requested.
> > > > >
> > > >
> > > > So, this should be the correct behavior.
> > > >
> > > >                  return vm_insert_range(vma, vma->vm_start,
> > > > rk_obj->pages + offset,
> > > >                                                           user_count);
> > >
> > > ... and by doing so, you're introducing another instance of the same
> > > bug I pointed out in patch 2.
> >
> > Sorry but didn't get it ? How it will be similar to the bug pointed
> > out in patch 2 ?
>

Thanks for the detail explanation.

> Towards the top of this function, you have:
>
>         unsigned long user_count = vma_pages(vma);
>
> So what you are proposing does:
>
>         return vm_insert_range(vma, vma->vm_start, rk_obj->pages + offset,
>                                vma_pages(vma));
>
> Now if we look inside vm_insert_range():
>
> +int vm_insert_range(struct vm_area_struct *vma, unsigned long addr,
> +                       struct page **pages, unsigned long page_count)
> +{
> +       unsigned long uaddr = addr;
> +       int ret = 0, i;
> +
> +       if (page_count > vma_pages(vma))
> +               return -ENXIO;
> +
> +       for (i = 0; i < page_count; i++) {
> +               ret = vm_insert_page(vma, uaddr, pages[i]);
> +               if (ret < 0)
> +                       return ret;
> +               uaddr += PAGE_SIZE;
> +       }
>
> So, page_count _is_ vma_pages(vma).  So this code does these operations:
>
>         if (vma_pages(vma) > vma_pages(vma))
>                 return -ENXIO;
>
> This will always be false.  I've already stated in my reply to patch 2
> in paragraph 3 about the uselessness of this test.

Agree, this will be always false for this particular/ similar instances.
But there are places [3/9], [6/9], [9/9] where page_count is already set
and it might be good to just cross check page_count > vma_pages(vma).

This was discussed during review of v3 [1/9].
https://patchwork.kernel.org/patch/10716601/

We can discuss again and if not needed it can be removed in v5.

>
>         for (i = 0; i < vma_pages(vma); i++) {
>                 ret = vm_insert_page(vma, uaddr, pages[i]);
>
> So the loop will iterate over the number of pages that the user requested.
>
> Now, taking another example.  The object is again 8 pages long, so
> indexes 0 through 7 in its page array are valid.  The user requests
> 8 pages at offset 2 into the object.  Also as already stated in
> paragraph 3 of my reply to patch 2.
>
> vma_pages(vma) is 8.  offset = 2.
>
> So we end up _inside_ vm_insert_range() with:
>
>         if (8 > 8)
>                 return -ENXIO;
>
> As stated, always false.
>
>         for (i = 0; i < 8; i++) {
>                 ret = vm_insert_page(vma, vaddr, rk_obj->pages[2 + i]);
>
> Which means we iterate over rk_obj->pages indicies from 2 through 9
> inclusive.
>
> Since only 0 through 7 are valid, we have walked off the end of the
> array, and attempted to map an invalid struct page pointer - we could
> be lucky, and it could point at some struct page (potentially causing
> us to map some sensitive page - maybe containing your bank details or
> root password... Or it could oops the kernel.

Consider the 2nd example.
The object is again 8 pages long, so indexes 0 through 7 in
its page array are valid.  The user requests 8 pages at offset 2
into the object.

The original code look like -

             unsigned long user_count = vma_pages(vma); // 8
             unsigned long end = user_count + offset // 8 + 2 = 10
              ...
              for (i = offset (2) ; i < end ( 10) ; i++) {
                  ret = vm_insert_page(vma, uaddr, rk_obj->pages[i]);
                  if (ret)
                     return ret;
                  uaddr += PAGE_SIZE;
              }

we iterate over rk_obj->pages indices from 2 through 9.
Does it indicates the actual code have a bug when *offset != 0*.