Re: [PATCH v4 4/9] drm/rockchip/rockchip_drm_gem.c: Convert to use vm_insert_range

On Wed, Dec 19, 2018 at 3:02 PM Russell King - ARM Linux
[off-list ref] wrote:

On Wed, Dec 19, 2018 at 09:01:09AM +0530, Souptick Joarder wrote:

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On Tue, Dec 18, 2018 at 6:31 PM Russell King - ARM Linux
[off-list ref] wrote:

quoted

On Tue, Dec 18, 2018 at 06:24:29PM +0530, Souptick Joarder wrote:

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On Tue, Dec 18, 2018 at 6:03 PM Russell King - ARM Linux
[off-list ref] wrote:

quoted

On Tue, Dec 18, 2018 at 05:36:04PM +0530, Souptick Joarder wrote:

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On Tue, Dec 18, 2018 at 3:27 PM Russell King - ARM Linux
[off-list ref] wrote:

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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 :)

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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*.

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