drm/i915/gem: Fix Virtual Memory mapping boundaries calculation

commit 8bdd9ef7e9b1b2a73e394712b72b22055e0e26c3 upstream.

Calculating the size of the mapped area as the lesser value
between the requested size and the actual size does not consider
the partial mapping offset. This can cause page fault access.

Fix the calculation of the starting and ending addresses, the
total size is now deduced from the difference between the end and
start addresses.

Additionally, the calculations have been rewritten in a clearer
and more understandable form.

Fixes: c58305af ("drm/i915: Use remap_io_mapping() to prefault all PTE in a single pass")
Reported-by: Jann Horn <jannh@google.com>
Co-developed-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: <stable@vger.kernel.org> # v4.9+
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Jonathan Cavitt <Jonathan.cavitt@intel.com>
[Joonas: Add Requires: tag]
Requires: 60a2066c5005 ("drm/i915/gem: Adjust vma offset for framebuffer mmap offset")
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240802083850.103694-3-andi.shyti@linux.intel.com


(cherry picked from commit 97b6784753da06d9d40232328efc5c5367e53417)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 3e06073d24807f04b4694108a8474decb7b99e60)
[Vegard: resolve conflict due to missing commit
 a65adaf8a834504a4acdc0deca7fa790771add8a ("drm/i915: Track user GTT
 faulting per-vma") from v4.15 and commit 73ebd503034c1abe ("drm/i915:
 make mappable struct resource centric") which motivates the start ->
 mappable_base and iomap -> mappable changes. Remove 'goto err_fence' as
 we have nothing else to do on failure; it should be OK to leave the GEM
 object on the mm's userfaultfd list.]
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>