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Re: [PATCH] KVM: fix the handling of dirty bitmaps to avoid overflows

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On 04/13/2010 03:52 AM, Takuya Yoshikawa wrote:
(2010/04/13 2:39), Marcelo Tosatti wrote:
On Mon, Apr 12, 2010 at 07:35:35PM +0900, Takuya Yoshikawa wrote:
This patch fixes a bug found by Avi during the review process
of my dirty bitmap related work.

To ppc and ia64 people:
   The fix is really simple but touches all architectures using
   dirty bitmaps. So please check this will not suffer your part.

===

Int is not long enough to store the size of a dirty bitmap.

This patch fixes this problem with the introduction of a wrapper
function to calculate the sizes of dirty bitmaps.

Note: in mark_page_dirty(), we have to consider the fact that
   __set_bit() takes the offset as int, not long.

Signed-off-by: Takuya Yoshikawa<yoshikawa.takuya@xxxxxxxxxxxxx>

Applied, thanks.


Thanks everyone!

BTW, just from my curiosity, are there any cases in which we use such huge
number of pages currently?

  ALIGN(memslot->npages, BITS_PER_LONG) / 8;

More than G pages need really big memory!
  -- We are assuming some special cases like "short" int size?

No, int is 32 bits, but memslot->npages is not our under control.

Note that you don't actually need all those pages to create a large memory slot.


If so, we may have to care about a lot of things from now on, because common
functions like __set_bit() don't support such long buffers.

It's better to limit memory slots to something that can be handled by everything, then. 2^31 pages is plenty. Return -EINVAL if the slot is too large.


--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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