kern/60907: i915: partial page view indexing error

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>Number:         60907
>Category:       kern
>Synopsis:       i915: partial page view indexing error
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Oct 20 13:05:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10
>Organization:
The PartialGGTT Overrun, Inc.
>Environment:
>Description:

	The logic in intel_partial_pages takes an object with a DMA map
	for pages [0,n) and a view [offset,offset+size) with

		0 <= offset <= offet + size <= n

	and creates a DMA map just for pages [offset,offset+size).

	However, owing to an indexing error, it may create some bogus
	addresses past the end of one of the segments.

	In particular, if the object has a segment {.ds_addr, .ds_len}
	for the left part of pages [offset,offset+size) so that the
	partial view requires taking the right part from the next
	segment, some of the pages in the middle might be instead taken
	to be .ds_addr + .ds_len, .ds_addr + .ds_len + PAGE_SIZE,
	.ds_addr + .ds_len + 2*PAGE_SIZE, and so on.

>How-To-Repeat:

	code inspection

>Fix:

-			    k < iseg->ds_len >> PAGE_SHIFT);
+			    k < (iseg->ds_len - offset) >> PAGE_SHIFT);

	plus some assertions to sanity-check the arithmetic