drm/amdgpu: prevent NULL pointer dereference if ATIF is not supported

acpi_evaluate_object() may return AE_NOT_FOUND (failure), which
would result in dereferencing buffer.pointer (obj) while being NULL.

Although this case may be unrealistic for the current code, it is
still better to protect against possible bugs.

Bail out also when status is AE_NOT_FOUND.

This fixes 1 FORWARD_NULL issue reported by Coverity
Report: CID 1600951:  Null pointer dereferences  (FORWARD_NULL)

Signed-off-by: Antonio Quartulli <[email protected]>
Fixes: c9b7c80 ("drm/amd: Guard against bad data for ATIF ACPI method")
Reviewed-by: Mario Limonciello <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mario Limonciello <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>
(cherry picked from commit 91c9e22)
Cc: [email protected]

Author: ordex

drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c

@@ -172,8 +172,8 @@ static union acpi_object *amdgpu_atif_call(struct amdgpu_atif *atif,
 				      &buffer);
 	obj = (union acpi_object *)buffer.pointer;
 
-	/* Fail if calling the method fails and ATIF is supported */
-	if (ACPI_FAILURE(status) && status != AE_NOT_FOUND) {
+	/* Fail if calling the method fails */
+	if (ACPI_FAILURE(status)) {
 		DRM_DEBUG_DRIVER("failed to evaluate ATIF got %s\n",
 				 acpi_format_exception(status));
 		kfree(obj);