DRA API: implement CEL cost limit

The main purpose is to protect against denial-of-service attacks.  Scheduling
time depends a lot on unpredictable factors and expected scheduling time also
varies, so no attempt is made to limit the overall time spent on evaluating CEL
expressions per claim.

Author: pohly

api/openapi-spec/swagger.json

@@ -143,7 +143,7 @@
         "properties": {
           "expression": {
             "default": "",
-            "description": "Expression is a CEL expression which evaluates a single device. It must evaluate to true when the device under consideration satisfies the desired criteria, and false when it does not. Any other result is an error and causes allocation of devices to abort.\n\nThe expression's input is an object named \"device\", which carries the following properties:\n - driver (string): the name of the driver which defines this device.\n - attributes (map[string]object): the device's attributes, grouped by prefix\n   (e.g. device.attributes[\"dra.example.com\"] evaluates to an object with all\n   of the attributes which were prefixed by \"dra.example.com\".\n - capacity (map[string]object): the device's capacities, grouped by prefix.\n\nExample: Consider a device with driver=\"dra.example.com\", which exposes two attributes named \"model\" and \"ext.example.com/family\" and which exposes one capacity named \"modules\". This input to this expression would have the following fields:\n\n    device.driver\n    device.attributes[\"dra.example.com\"].model\n    device.attributes[\"ext.example.com\"].family\n    device.capacity[\"dra.example.com\"].modules\n\nThe device.driver field can be used to check for a specific driver, either as a high-level precondition (i.e. you only want to consider devices from this driver) or as part of a multi-clause expression that is meant to consider devices from different drivers.\n\nThe value type of each attribute is defined by the device definition, and users who write these expressions must consult the documentation for their specific drivers. The value type of each capacity is Quantity.\n\nIf an unknown prefix is used as a lookup in either device.attributes or device.capacity, an empty map will be returned. Any reference to an unknown field will cause an evaluation error and allocation to abort.\n\nA robust expression should check for the existence of attributes before referencing them.\n\nFor ease of use, the cel.bind() function is enabled, and can be used to simplify expressions that access multiple attributes with the same domain. For example:\n\n    cel.bind(dra, device.attributes[\"dra.example.com\"], dra.someBool && dra.anotherBool)",
+            "description": "Expression is a CEL expression which evaluates a single device. It must evaluate to true when the device under consideration satisfies the desired criteria, and false when it does not. Any other result is an error and causes allocation of devices to abort.\n\nThe expression's input is an object named \"device\", which carries the following properties:\n - driver (string): the name of the driver which defines this device.\n - attributes (map[string]object): the device's attributes, grouped by prefix\n   (e.g. device.attributes[\"dra.example.com\"] evaluates to an object with all\n   of the attributes which were prefixed by \"dra.example.com\".\n - capacity (map[string]object): the device's capacities, grouped by prefix.\n\nExample: Consider a device with driver=\"dra.example.com\", which exposes two attributes named \"model\" and \"ext.example.com/family\" and which exposes one capacity named \"modules\". This input to this expression would have the following fields:\n\n    device.driver\n    device.attributes[\"dra.example.com\"].model\n    device.attributes[\"ext.example.com\"].family\n    device.capacity[\"dra.example.com\"].modules\n\nThe device.driver field can be used to check for a specific driver, either as a high-level precondition (i.e. you only want to consider devices from this driver) or as part of a multi-clause expression that is meant to consider devices from different drivers.\n\nThe value type of each attribute is defined by the device definition, and users who write these expressions must consult the documentation for their specific drivers. The value type of each capacity is Quantity.\n\nIf an unknown prefix is used as a lookup in either device.attributes or device.capacity, an empty map will be returned. Any reference to an unknown field will cause an evaluation error and allocation to abort.\n\nA robust expression should check for the existence of attributes before referencing them.\n\nFor ease of use, the cel.bind() function is enabled, and can be used to simplify expressions that access multiple attributes with the same domain. For example:\n\n    cel.bind(dra, device.attributes[\"dra.example.com\"], dra.someBool && dra.anotherBool)\n\nThe length of the expression must be smaller or equal to 10 Ki. The cost of evaluating it is also limited based on the estimated number of logical steps.",
             "type": "string"
           }
         },

api/openapi-spec/v3/apis__resource.k8s.io__v1alpha3_openapi.json

@@ -526,10 +526,37 @@ type CELDeviceSelector struct {
 	//
 	//     cel.bind(dra, device.attributes["dra.example.com"], dra.someBool && dra.anotherBool)
 	//
+	// The length of the expression must be smaller or equal to 10 Ki. The
+	// cost of evaluating it is also limited based on the estimated number
+	// of logical steps.
+	//
 	// +required
 	Expression string
 }
 
+// CELSelectorExpressionMaxCost specifies the cost limit for a single CEL selector
+// evaluation.
+//
+// There is no overall budget for selecting a device, so the actual time
+// required for that is proportional to the number of CEL selectors and how
+// often they need to be evaluated, which can vary depending on several factors
+// (number of devices, cluster utilization, additional constraints).
+//
+// Validation against this limit and [CELSelectorExpressionMaxLength] happens
+// only when setting an expression for the first time or when changing it. If
+// the limits are changed in a future Kubernetes release, existing users are
+// guaranteed that existing expressions will continue to be valid and won't be
+// interrupted at runtime after an up- or downgrade.
+//
+// According to
+// https://github.com/kubernetes/kubernetes/blob/4aeaf1e99e82da8334c0d6dddd848a194cd44b4f/staging/src/k8s.io/apiserver/pkg/apis/cel/config.go#L20-L22,
+// this gives roughly 0.1 second for each expression evaluation.
+// However, this depends on how fast the machine is.
+const CELSelectorExpressionMaxCost = 1000000
+
+// CELSelectorExpressionMaxLength is the maximum length of a CEL selector expression string.
+const CELSelectorExpressionMaxLength = 10 * 1024
+
 // DeviceConstraint must have exactly one field set besides Requests.
 type DeviceConstraint struct {
 	// Requests is a list of the one or more requests in this claim which

pkg/apis/resource/types.go

@@ -170,10 +170,19 @@ func validateCELSelector(celSelector resource.CELDeviceSelector, fldPath *field.
 	if stored {
 		envType = environment.StoredExpressions
 	}
+	if len(celSelector.Expression) > resource.CELSelectorExpressionMaxLength {
+		allErrs = append(allErrs, field.TooLongMaxLength(fldPath.Child("expression"), "<value omitted>", resource.CELSelectorExpressionMaxLength))
+		// Don't bother compiling too long expressions.
+		return allErrs
+	}
+
 	result := dracel.GetCompiler().CompileCELExpression(celSelector.Expression, envType)
 	if result.Error != nil {
 		allErrs = append(allErrs, convertCELErrorToValidationError(fldPath.Child("expression"), celSelector.Expression, result.Error))
+	} else if result.MaxCost > resource.CELSelectorExpressionMaxCost {
+		allErrs = append(allErrs, field.Forbidden(fldPath.Child("expression"), "too complex, exceeds cost limit"))
 	}
+
 	return allErrs
 }
 

pkg/apis/resource/validation/validation.go

@@ -18,6 +18,7 @@ package validation
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -316,6 +317,48 @@ func TestValidateClaim(t *testing.T) {
 				return claim
 			}(),
 		},
+		"CEL-length": {
+			wantFailures: field.ErrorList{
+				field.TooLongMaxLength(field.NewPath("spec", "devices", "requests").Index(1).Child("selectors").Index(1).Child("cel", "expression"), "<value omitted>", resource.CELSelectorExpressionMaxLength),
+			},
+			claim: func() *resource.ResourceClaim {
+				claim := testClaim(goodName, goodNS, validClaimSpec)
+				claim.Spec.Devices.Requests = append(claim.Spec.Devices.Requests, claim.Spec.Devices.Requests[0])
+				claim.Spec.Devices.Requests[1].Name += "-2"
+				expression := `device.driver == ""`
+				claim.Spec.Devices.Requests[1].Selectors = []resource.DeviceSelector{
+					{
+						// Good selector.
+						CEL: &resource.CELDeviceSelector{
+							Expression: strings.ReplaceAll(expression, `""`, `"`+strings.Repeat("x", resource.CELSelectorExpressionMaxLength-len(expression))+`"`),
+						},
+					},
+					{
+						// Too long by one selector.
+						CEL: &resource.CELDeviceSelector{
+							Expression: strings.ReplaceAll(expression, `""`, `"`+strings.Repeat("x", resource.CELSelectorExpressionMaxLength-len(expression)+1)+`"`),
+						},
+					},
+				}
+				return claim
+			}(),
+		},
+		"CEL-cost": {
+			wantFailures: field.ErrorList{
+				field.Forbidden(field.NewPath("spec", "devices", "requests").Index(0).Child("selectors").Index(0).Child("cel", "expression"), "too complex, exceeds cost limit"),
+			},
+			claim: func() *resource.ResourceClaim {
+				claim := testClaim(goodName, goodNS, validClaimSpec)
+				claim.Spec.Devices.Requests[0].Selectors = []resource.DeviceSelector{
+					{
+						CEL: &resource.CELDeviceSelector{
+							Expression: `device.attributes["dra.example.com"].map(s, s.lowerAscii()).map(s, s.size()).sum() == 0`,
+						},
+					},
+				}
+				return claim
+			}(),
+		},
 	}
 
 	for name, scenario := range scenarios {