[PATCH] drm/qxl: remove double page-size scaling in kmap fallback paths

[PATCH] drm/qxl: remove double page-size scaling in kmap fallback paths

[Aldo Ariel Panzardo]

qxl_bo_kmap_atomic_page() takes a page_offset parameter that callers
compute as a byte offset aligned to a page boundary:

    reloc_page = qxl_bo_kmap_atomic_page(qdev, info->dst_bo,
                                         info->dst_offset & PAGE_MASK);

The VRAM/PRIV path adds it directly to the mapping base (both in
bytes), which is correct:

    offset = bo->tbo.resource->start << PAGE_SHIFT;
    return io_mapping_map_atomic_wc(map, offset + page_offset);

But the two system-memory fallback paths multiply it by PAGE_SIZE
again:

    rptr = bo->kptr + (page_offset * PAGE_SIZE);

For page_offset = 0x5000 (page 5), this computes
0x5000 * 0x1000 = 0x5000000, a 80 MiB offset instead of 20 KiB,
producing a massive out-of-bounds access into kernel memory.

Remove the spurious multiplication so the fallback paths use the
same byte-offset semantics as the io_mapping path.

Fixes: f64122c1f6ad ("drm: add new QXL driver. (v1.4)")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
---
 drivers/gpu/drm/qxl/qxl_object.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/qxl/qxl_object.c b/drivers/gpu/drm/qxl/qxl_object.c
index XXXXXXX..YYYYYYY 100644
--- a/drivers/gpu/drm/qxl/qxl_object.c
+++ b/drivers/gpu/drm/qxl/qxl_object.c
@@ -223,7 +223,7 @@ void *qxl_bo_kmap_atomic_page(struct qxl_device *qdev,
 	return io_mapping_map_atomic_wc(map, offset + page_offset);
 fallback:
 	if (bo->kptr) {
-		rptr = bo->kptr + (page_offset * PAGE_SIZE);
+		rptr = bo->kptr + page_offset;
 		return rptr;
 	}

@@ -232,7 +232,7 @@ void *qxl_bo_kmap_atomic_page(struct qxl_device *qdev,
 		return NULL;
 	rptr = bo_map.vaddr; /* TODO: Use mapping abstraction properly */

-	rptr += page_offset * PAGE_SIZE;
+	rptr += page_offset;
 	return rptr;
 }

--
2.43.0