drm/amd/display: fix a Null pointer dereference vulnerability

[Why]
A null pointer dereference vulnerability exists in the AMD display driver's
(DC module) cleanup function dc_destruct().
When display control context (dc->ctx) construction fails
(due to memory allocation failure), this pointer remains NULL.
During subsequent error handling when dc_destruct() is called,
there's no NULL check before dereferencing the perf_trace member
(dc->ctx->perf_trace), causing a kernel null pointer dereference crash.

[How]
Check if dc->ctx is non-NULL before dereferencing.

Link: https://lore.kernel.org/r/[email protected]
Co-developed-by: Mario Limonciello <[email protected]>
Signed-off-by: Mario Limonciello <[email protected]>
(Updated commit text and removed unnecessary error message)
Signed-off-by: Siyang Liu <[email protected]>
Signed-off-by: Roman Li <[email protected]>
Reviewed-by: Alex Hung <[email protected]>
Tested-by: Daniel Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

Author: TencentAISec

drivers/gpu/drm/amd/display/dc/core/dc.c

@@ -940,17 +940,18 @@ static void dc_destruct(struct dc *dc)
 	if (dc->link_srv)
 		link_destroy_link_service(&dc->link_srv);
 
-	if (dc->ctx->gpio_service)
-		dal_gpio_service_destroy(&dc->ctx->gpio_service);
+	if (dc->ctx) {
+		if (dc->ctx->gpio_service)
+			dal_gpio_service_destroy(&dc->ctx->gpio_service);
 
-	if (dc->ctx->created_bios)
-		dal_bios_parser_destroy(&dc->ctx->dc_bios);
+		if (dc->ctx->created_bios)
+			dal_bios_parser_destroy(&dc->ctx->dc_bios);
+		kfree(dc->ctx->logger);
+		dc_perf_trace_destroy(&dc->ctx->perf_trace);
 
-	kfree(dc->ctx->logger);
-	dc_perf_trace_destroy(&dc->ctx->perf_trace);
-
-	kfree(dc->ctx);
-	dc->ctx = NULL;
+		kfree(dc->ctx);
+		dc->ctx = NULL;
+	}
 
 	kfree(dc->bw_vbios);
 	dc->bw_vbios = NULL;