SQL Server Programming and Host Protection Attributes 

The ability to load and execute managed code in a SQL Server host requires meeting the host's requirements for both code access security and host resource protection. The code access security requirements are specified by one of three SQL Server permission sets: SAFE, EXTERNAL-ACCESS, or UNSAFE. Code executing within the SAFE or EXTERNAL-ACCESS permission sets must avoid certain types or members that have the HostProtectionAttribute attribute applied. The HostProtectionAttribute is not a security permission as much as a reliability guarantee in that it identifies specific code constructs, either types or methods, that the host may disallow. The use of the HostProtectionAttribute enforces a programming model that helps protect the stability of the host.

Host Protection Attributes

Host protection attributes identify types or members that do not fit the host programming model and represent the following increasing levels of reliability threat:

• Are otherwise benign.

• Could lead to destabilization of server-managed user code.

• Could lead to destabilization of the server process itself.

SQL Server disallows the use of a type or member that has a HostProtectionAttribute that specifies a HostProtectionResource value of SharedState, Synchronization, MayLeakOnAbort, or ExternalProcessMgmt. This prevents the assemblies from calling members that enable sharing state, perform synchronization, might cause a resource leak on termination, or affect the integrity of the SQL Server process.

Disallowed Types and Members

The following table identifies types and members whose HostProtectionResource values are disallowed by SQL Server.

Namespace	Type or member
Microsoft.Win32	PowerModeChangedEventArgs class PowerModeChangedEventHandler delegate SessionEndedEventArgs class SessionEndedEventHandler delegate SessionEndingEventArgs class SessionEndingEventHandler delegate SessionSwitchEventArgs class SessionSwitchEventHandler delegate SystemEvents class TimerElapsedEventArgs class TimerElapsedEventHandler delegate UserPreferenceChangedEventArgs class UserPreferenceChangingEventArgs class
System.Collections	System.Collections.ArrayList.Synchronized method System.Collections.Hashtable.Synchronized(System.Collections.Hashtable) method System.Collections.Queue.Synchronized(System.Collections.Queue) method System.Collections.SortedList.Synchronized(System.Collections.SortedList) method System.Collections.Stack.Synchronized(System.Collections.Stack) method
System.ComponentModel	AddingNewEventArgs class AddingNewEventHandler delegate ArrayConverter class AsyncCompletedEventArgs class AsyncCompletedEventHandler delegate AsyncOperation class AsyncOperationManager class AttributeCollection class BackgroundWorker class BaseNumberConverter class BindingList class BooleanConverter class ByteConverter class CancelEventArgs class CancelEventHandler delegate CharConverter class CollectionChangeEventArgs class CollectionChangeEventHandler delegate CollectionConverter class ComponentCollection class ComponentConverter class ComponentEditor class ComponentResourceManager class Container class ContainerFilterService class CultureInfoConverter class CustomTypeDescriptor class DateTimeConverter class DecimalConverter class ActiveDesignerEventArgs class ActiveDesignerEventHandler delegate CheckoutException class CommandID class ComponentChangedEventArgs class ComponentChangedEventHandler delegate ComponentChangingEventArgs class ComponentChangingEventHandler delegate ComponentEventArgs class ComponentEventHandler delegate ComponentRenameEventArgs class ComponentRenameEventHandler delegate DesignerCollection class DesignerEventArgs class DesignerEventHandler delegate DesignerOptionService class DesignerTransaction class DesignerTransactionCloseEventArgs class DesignerTransactionCloseEventHandler delegate DesignerVerb class DesignerVerbCollection class DesigntimeLicenseContext class DesigntimeLicenseContextSerializer class MenuCommand class ComponentSerializationService class ContextStack class DesignerLoader class InstanceDescriptor class MemberRelationshipService class ResolveNameEventArgs class ResolveNameEventHandler delegate SerializationStore class ServiceContainer class ServiceCreatorCallback delegate StandardCommands class StandardToolWindows class DoubleConverter class DoWorkEventArgs class DoWorkEventHandler delegate EnumConverter class EventDescriptor class EventDescriptorCollection class EventHandlerList class ExpandableObjectConverter class HandledEventArgs class HandledEventHandler delegate InstanceCreationEditor class Int16Converter class Int32Converter class Int64Converter class InvalidAsynchronousStateException class InvalidEnumArgumentException class BeginInvoke method License class LicenseContext class LicenseException class LicenseManager class LicenseProvider class LicFileLicenseProvider class ListChangedEventArgs class ListChangedEventHandler delegate ListSortDescription class ListSortDescriptionCollection class MaskedTextProvider class MemberDescriptor class MultilineStringConverter class NestedContainer class NullableConverter class ProgressChangedEventArgs class ProgressChangedEventHandler delegate PropertyChangedEventArgs class PropertyChangedEventHandler delegate PropertyDescriptor class PropertyDescriptorCollection class ReferenceConverter class RefreshEventArgs class RefreshEventHandler delegate RunWorkerCompletedEventArgs class RunWorkerCompletedEventHandler delegate SByteConverter class SingleConverter class StringConverter class SyntaxCheck class TimeSpanConverter class TypeConverter class TypeDescriptionProvider class TypeDescriptor class TypeListConverter class UInt16Converter class UInt32Converter class UInt64Converter class WarningException class Win32Exception class
System.Diagnostics	System.Diagnostics.Debug.Listeners property System.Diagnostics.Trace.Listeners property System.Diagnostics.EventLog.SynchronizingObject property ConsoleTraceListener class DefaultTraceListener class DelimitedListTraceListener class EventLogTraceListener class PerformanceCounter class PerformanceCounterCategory class Process class ProcessStartInfo class TextWriterTraceListener class TraceListener class XmlWriterTraceListener class System.Diagnostics.TraceSource.Listeners property
System.IO	System.IO.Stream.Synchronized(System.IO.Stream) method System.IO.TextReader.Synchronized(System.IO.TextReader) method System.IO.TextWriter.Synchronized(System.IO.TextWriter) method
System.Reflection.Emit	ConstructorBuilder class EventBuilder class FieldBuilder class MethodBuilder class CustomAttributeBuilder class MethodRental class ModuleBuilder class PropertyBuilder class TypeBuilder class UnmanagedMarshal class
System.Text	System.Text.RegularExpressions.Group.Synchronized(System.Text.RegularExpressions.Group) method System.Text.RegularExpressions.Match.Synchronized(System.Text.RegularExpressions.Match) method
System.Threading	AutoResetEvent class EventWaitHandle class ManualResetEvent class Monitor class Mutex class ReaderWriterLock class Semaphore class System.Threading.Thread.AllocateNamedDataSlot(System.String) method System.Threading.Thread.BeginCriticalRegion method System.Threading.Thread.EndCriticalRegion method System.Threading.Thread.FreeNamedDataSlot(System.String) method System.Threading.Thread.GetData(System.LocalDataStoreSlot) method System.Threading.Thread.Join method System.Threading.Thread.SetApartmentState(System.Threading.ApartmentState) method System.Threading.Thread.SetData(System.LocalDataStoreSlot,System.Object) method System.Threading.Thread.SpinWait(System.Int32) method System.Threading.Thread.Start method System.Threading.Thread.TrySetApartmentState(System.Threading.ApartmentState) method ThreadPool class Timer class
System.Timers	Timer class
System.Web.Configuration	MachineKeyValidationConverter class
System.Windows.Forms	System.Windows.Forms.AutoCompleteStringCollection.SyncRoot property

SQL Server Permission Sets

SQL Server allows users to specify the reliability requirements for code deployed into a database. When assemblies are uploaded into the database, the author of the assembly can specify one of three permission sets for that assembly: SAFE, EXTERNAL-ACCESS, or UNSAFE.

Permission set	SAFE	EXTERNAL-ACCESS	UNSAFE
Code access security	Execute only	Execute + access to external resources	Unrestricted
Programming model restrictions	Yes	Yes	No restrictions
Verifiability requirement	Yes	Yes	No
Ability to call native code	No	No	Yes

SAFE is the most reliable and secure mode with associated restrictions in terms of the allowed programming model. SAFE code has high reliability and security features. SAFE assemblies are given enough permission to run, perform computations, and have access to the local database. SAFE assemblies need to be verifiably type safe and are not allowed to call unmanaged code.

EXTERNAL-ACCESS provides an intermediate security option, allowing code to access resources external to the database but still having the reliability and safety of SAFE.

UNSAFE is for highly trusted code that can only be created by database administrators. This trusted code has no code access restrictions, and it can call unmanaged (native) code.

SQL Server uses the host-level code access security policy layer to set up a host policy that grants one of the three sets of permissions based on the permission set stored in SQL Server catalogs. Managed code running inside the database always gets one of these code access permission sets.

Programming Model Restrictions

The programming model for managed code in SQL Server requires functions, procedures, and types which do not require the use of state held across multiple invocations or the sharing of state across multiple user sessions. Further, as described earlier, the presence of shared state can cause critical exceptions that impact the scalability and the reliability of the application.

Given these considerations, SQL Server disallows the use of static variables and static data members. For SAFE and EXTERNAL-ACCESS assemblies, SQL Server examines the metadata of the assembly at CREATE ASSEMBLY time, and fails the creation of such assemblies if it finds the use of static data members and variables.

See Also

Reference

HostProtectionAttribute
HostProtectionResource