drm/i915: Avoid potential vm use-after-free

Adding the vm to the vm_xa table makes it visible to userspace, which
could try to race with us to close the vm.  So we need to take our extra
reference before putting it in the table.
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Fixes: 9ec8795e ("drm/i915: Drop __rcu from gem_context->vm")
Cc: <stable@vger.kernel.org> # v5.16+
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230119173321.2825472-1-robdclark@gmail.com

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -1860,11 +1860,19 @@ static int get_ppgtt(struct drm_i915_file_private *file_priv,
 	vm = ctx->vm;
 	GEM_BUG_ON(!vm);
 
+	/*
+	 * Get a reference for the allocated handle.  Once the handle is
+	 * visible in the vm_xa table, userspace could try to close it
+	 * from under our feet, so we need to hold the extra reference
+	 * first.
+	 */
+	i915_vm_get(vm);
+
 	err = xa_alloc(&file_priv->vm_xa, &id, vm, xa_limit_32b, GFP_KERNEL);
-	if (err)
+	if (err) {
+		i915_vm_put(vm);
 		return err;
-
-	i915_vm_get(vm);
+	}
 
 	GEM_BUG_ON(id == 0); /* reserved for invalid/unassigned ppgtt */
 	args->value = id;