drm/i915/guc: Fix potential invalid pointer dereferences when decodin…

…g G2Hs

Some G2H handlers were reading the context id field from the payload
before checking the payload met the minimum length required.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220302003357.4188363-9-John.C.Harrison@Intel.com

drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c

@@ -3895,12 +3895,13 @@ int intel_guc_deregister_done_process_msg(struct intel_guc *guc,
 					  u32 len)
 {
 	struct intel_context *ce;
-	u32 ctx_id = msg[0];
+	u32 ctx_id;
 
 	if (unlikely(len < 1)) {
 		drm_err(&guc_to_gt(guc)->i915->drm, "Invalid length %u\n", len);
 		return -EPROTO;
 	}
+	ctx_id = msg[0];
 
 	ce = g2h_context_lookup(guc, ctx_id);
 	if (unlikely(!ce))
@@ -3946,12 +3947,13 @@ int intel_guc_sched_done_process_msg(struct intel_guc *guc,
 {
 	struct intel_context *ce;
 	unsigned long flags;
-	u32 ctx_id = msg[0];
+	u32 ctx_id;
 
 	if (unlikely(len < 2)) {
 		drm_err(&guc_to_gt(guc)->i915->drm, "Invalid length %u\n", len);
 		return -EPROTO;
 	}
+	ctx_id = msg[0];
 
 	ce = g2h_context_lookup(guc, ctx_id);
 	if (unlikely(!ce))