drm/vmwgfx: Prevent unmapping active read buffers

[Zack Rusin <zack.rusin@xxxxxxxxxxxx>]

From: Zack Rusin <zack.rusin@xxxxxxxxxxxx>

commit aba07b9a0587f50e5d3346eaa19019cf3f86c0ea upstream.

The kms paths keep a persistent map active to read and compare the cursor
buffer. These maps can race with each other in simple scenario where:
a) buffer "a" mapped for update
b) buffer "a" mapped for compare
c) do the compare
d) unmap "a" for compare
e) update the cursor
f) unmap "a" for update
At step "e" the buffer has been unmapped and the read contents is bogus.

Prevent unmapping of active read buffers by simply keeping a count of
how many paths have currently active maps and unmap only when the count
reaches 0.

Fixes: 485d98d472d5 ("drm/vmwgfx: Add support for CursorMob and CursorBypass 4")
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@xxxxxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx> # v5.19+
Signed-off-by: Zack Rusin <zack.rusin@xxxxxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20240816183332.31961-2-zack.rusin@xxxxxxxxxxxx
Reviewed-by: Martin Krastev <martin.krastev@xxxxxxxxxxxx>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_bo.c |   13 +++++++++++--
 drivers/gpu/drm/vmwgfx/vmwgfx_bo.h |    3 +++
 2 files changed, 14 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
@@ -360,6 +360,8 @@ void *vmw_bo_map_and_cache_size(struct v
 	void *virtual;
 	int ret;
 
+	atomic_inc(&vbo->map_count);
+
 	virtual = ttm_kmap_obj_virtual(&vbo->map, &not_used);
 	if (virtual)
 		return virtual;
@@ -383,11 +385,17 @@ void *vmw_bo_map_and_cache_size(struct v
  */
 void vmw_bo_unmap(struct vmw_bo *vbo)
 {
+	int map_count;
+
 	if (vbo->map.bo == NULL)
 		return;
 
-	ttm_bo_kunmap(&vbo->map);
-	vbo->map.bo = NULL;
+	map_count = atomic_dec_return(&vbo->map_count);
+
+	if (!map_count) {
+		ttm_bo_kunmap(&vbo->map);
+		vbo->map.bo = NULL;
+	}
 }
 
 
@@ -421,6 +429,7 @@ static int vmw_bo_init(struct vmw_privat
 	vmw_bo->tbo.priority = 3;
 	vmw_bo->res_tree = RB_ROOT;
 	xa_init(&vmw_bo->detached_resources);
+	atomic_set(&vmw_bo->map_count, 0);
 
 	params->size = ALIGN(params->size, PAGE_SIZE);
 	drm_gem_private_object_init(vdev, &vmw_bo->tbo.base, params->size);
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h
@@ -71,6 +71,8 @@ struct vmw_bo_params {
  * @map: Kmap object for semi-persistent mappings
  * @res_tree: RB tree of resources using this buffer object as a backing MOB
  * @res_prios: Eviction priority counts for attached resources
+ * @map_count: The number of currently active maps. Will differ from the
+ * cpu_writers because it includes kernel maps.
  * @cpu_writers: Number of synccpu write grabs. Protected by reservation when
  * increased. May be decreased without reservation.
  * @dx_query_ctx: DX context if this buffer object is used as a DX query MOB
@@ -90,6 +92,7 @@ struct vmw_bo {
 	u32 res_prios[TTM_MAX_BO_PRIORITY];
 	struct xarray detached_resources;
 
+	atomic_t map_count;
 	atomic_t cpu_writers;
 	/* Not ref-counted.  Protected by binding_mutex */
 	struct vmw_resource *dx_query_ctx;


Patches currently in stable-queue which might be from zack.rusin@xxxxxxxxxxxx are

queue-6.10/drm-vmwgfx-prevent-unmapping-active-read-buffers.patch
queue-6.10/drm-vmwgfx-disable-coherent-dumb-buffers-without-3d.patch
queue-6.10/drm-vmwgfx-fix-prime-with-external-buffers.patch