ASP.NET Core 認證與受權[6]:受權策略是怎麼執行的？

原文: ASP.NET Core 認證與受權[6]:受權策略是怎麼執行的？

在上一章中，詳細介紹了 ASP.NET Core 中的受權策略，在須要受權時，只須要在對應的Controler或者Action上面打上[Authorize]特性，並指定要執行的策略名稱便可，可是，受權策略是怎麼執行的呢？懷着一顆好奇的心，忍不住來探索一下它的執行流程。

目錄

1. MVC中的受權
   • AuthorizationApplicationModelProvider
   • AuthorizeFilter
2. IPolicyEvaluator
   • AuthenticateAsync(AuthenticationSchemes)
   • AuthorizeAsync(Requirements)
3. IAuthorizationService
   • IAuthorizationPolicyProvider
   • IAuthorizationHandlerContextFactory
   • IAuthorizationHandlerProvider
   • IAuthorizationEvaluator

在《(上一章》中提到，AuthorizeAttribute只是一個簡單的實現了IAuthorizeData接口的特性，而且在 ASP.NET Core 受權系統中並無使用到它。咱們知道在認證中，還有一個UseAuthentication擴展方法來激活認證系統，可是在受權中並無相似的機制。

這是由於當咱們使用[Authorize]一般是在MVC中，由MVC來負責激活受權系統。原本在這個系列的文章中，我並不想涉及到MVC的知識，可是爲了能更好的理解受權系統的執行，就來簡單介紹一下MVC中與受權相關的知識。

MVC中的受權

當咱們使用MVC時，首先會調用MVC的AddMvc擴展方法，用來註冊一些MVC相關的服務：

public static IMvcBuilder AddMvc(this IServiceCollection services)
{
    var builder = services.AddMvcCore();

    builder.AddAuthorization();

    ...
}

public static IMvcCoreBuilder AddAuthorization(this IMvcCoreBuilder builder)
{
    AddAuthorizationServices(builder.Services);
    return builder;
}

internal static void AddAuthorizationServices(IServiceCollection services)
{
    services.AddAuthenticationCore();
    services.AddAuthorization();
    services.AddAuthorizationPolicyEvaluator();

    services.TryAddEnumerable(
        ServiceDescriptor.Transient<IApplicationModelProvider, AuthorizationApplicationModelProvider>());
}

在上面AddAuthorizationServices中的前三個方法都屬於 ASP.NET Core 《Security》項目中提供的擴展方法，其中前兩個在前面幾章已經介紹過了，對於AddAuthorizationPolicyEvaluator放到後面再來介紹，咱們先來看一下MVC中的AuthorizationApplicationModelProvider。

AuthorizationApplicationModelProvider

在MVC中有一個ApplicationModel的概念，它用來封裝Controller, Filter, ApiExplorer等。對應的，在MVC中還提供了一系列的ApplicationModelProvider來初始化ApplicationModel的各個部分，而AuthorizationApplicationModelProvider就是用來初始化與受權相關的部分。

public class AuthorizationApplicationModelProvider : IApplicationModelProvider
{
    public void OnProvidersExecuting(ApplicationModelProviderContext context)
    {
        foreach (var controllerModel in context.Result.Controllers)
        {
            var controllerModelAuthData = controllerModel.Attributes.OfType<IAuthorizeData>().ToArray();
            if (controllerModelAuthData.Length > 0)
            {
                controllerModel.Filters.Add(GetFilter(_policyProvider, controllerModelAuthData));
            }
            foreach (var attribute in controllerModel.Attributes.OfType<IAllowAnonymous>())
            {
                controllerModel.Filters.Add(new AllowAnonymousFilter());
            }
            foreach (var actionModel in controllerModel.Actions)
            {
                var actionModelAuthData = actionModel.Attributes.OfType<IAuthorizeData>().ToArray();
                if (actionModelAuthData.Length > 0)
                {
                    actionModel.Filters.Add(GetFilter(_policyProvider, actionModelAuthData));
                }
                foreach (var attribute in actionModel.Attributes.OfType<IAllowAnonymous>())
                {
                    actionModel.Filters.Add(new AllowAnonymousFilter());
                }
            }
        }
    }
}

如上，首先查找每一個Controller中實現了IAuthorizeData接口的特性，而後將其轉化爲AuthorizeFilter並添加到Controller的Filter集合中，緊接着再查找實現了IAllowAnonymous接口的特性，將其轉化爲AllowAnonymousFilter過濾器也添加到Filter集合中，而後以一樣的邏輯查找Action上的特性並添加到Action的Filter集合中。

其中的關鍵點就是將IAuthorizeData（也就是經過咱們熟悉的[Authorize]特性）轉化爲MVC中的AuthorizeFilter過濾器：

public static AuthorizeFilter GetFilter(IAuthorizationPolicyProvider policyProvider, IEnumerable<IAuthorizeData> authData)
{
    if (policyProvider.GetType() == typeof(DefaultAuthorizationPolicyProvider))
    {
        var policy = AuthorizationPolicy.CombineAsync(policyProvider, authData).GetAwaiter().GetResult();
        return new AuthorizeFilter(policy);
    }
    else
    {
        return new AuthorizeFilter(policyProvider, authData);
    }
}

CombineAsync在上一章的《AuthorizationPolicy》中已經介紹過了，咱們往下看看AuthorizeFilter的實現。

AuthorizeFilter

在MVC中有一個AuthorizeFilter過濾器，相似咱們在ASP.NET 4.x中所熟悉的[Authorize]，它實現了IAsyncAuthorizationFilter接口，定義以下：

public class AuthorizeFilter : IAsyncAuthorizationFilter, IFilterFactory
{
    public AuthorizeFilter(AuthorizationPolicy policy) {}
    public AuthorizeFilter(IAuthorizationPolicyProvider policyProvider, IEnumerable<IAuthorizeData> authorizeData) : this(authorizeData) {}
    public AuthorizeFilter(IEnumerable<IAuthorizeData> authorizeData) {}

    public IEnumerable<IAuthorizeData> AuthorizeData { get; }
    public AuthorizationPolicy Policy { get; }

    public virtual async Task OnAuthorizationAsync(AuthorizationFilterContext context)
    {
        var effectivePolicy = Policy;
        if (effectivePolicy == null)
        {
            effectivePolicy = await AuthorizationPolicy.CombineAsync(PolicyProvider, AuthorizeData);
        }
        var policyEvaluator = context.HttpContext.RequestServices.GetRequiredService<IPolicyEvaluator>();
        var authenticateResult = await policyEvaluator.AuthenticateAsync(effectivePolicy, context.HttpContext);
        if (context.Filters.Any(item => item is IAllowAnonymousFilter))
        {
            return;
        }
        var authorizeResult = await policyEvaluator.AuthorizeAsync(effectivePolicy, authenticateResult, context.HttpContext, context);

        ... // 若是受權失敗，返回ChallengeResult或ForbidResult
    }
}

AuthorizeFilter的OnAuthorizationAsync方法會在Action執行以前觸發，其調用IPolicyEvaluator來完成受權，將執行流程切回到 ASP.NET Core 受權系統中。關於MVC中IApplicationModelProvider以及Filter的概念，在之後MVC系列的文章中再來詳細介紹，下面就繼續介紹 ASP.NET Core 的受權系統，也就是《Security》項目。

IPolicyEvaluator

IPolicyEvaluator是MVC調用受權系統的入口點，其定義以下：

public interface IPolicyEvaluator
{
    Task<AuthenticateResult> AuthenticateAsync(AuthorizationPolicy policy, HttpContext context);
    Task<PolicyAuthorizationResult> AuthorizeAsync(AuthorizationPolicy policy, AuthenticateResult authenticationResult, HttpContext context, object resource);
}

在上面介紹的AddMVC中，調用了AddAuthorizationPolicyEvaluator擴展方法，它有以下定義：

public static class PolicyServiceCollectionExtensions
{
    public static IServiceCollection AddAuthorizationPolicyEvaluator(this IServiceCollection services)
    {
        services.TryAdd(ServiceDescriptor.Transient<IPolicyEvaluator, PolicyEvaluator>());
        return services;
    }
}

由此可知IPolicyEvaluator的默認實現爲PolicyEvaluator，咱們就從它入手，來一步一步解剖 ASP.NET Core 受權系統的執行步驟。

在AuthorizeFilter中，依次調到了AuthenticateAsync和AuthorizeAsync方法，咱們就一一來看。

AuthenticateAsync(AuthenticationSchemes)

爲何還有一個AuthenticateAsync方法呢，這不是在認證階段執行的嗎？咱們看下它的實現：

public class PolicyEvaluator : IPolicyEvaluator
{
    public virtual async Task<AuthenticateResult> AuthenticateAsync(AuthorizationPolicy policy, HttpContext context)
    {
        if (policy.AuthenticationSchemes != null && policy.AuthenticationSchemes.Count > 0)
        {
            ClaimsPrincipal newPrincipal = null;
            foreach (var scheme in policy.AuthenticationSchemes)
            {
                var result = await context.AuthenticateAsync(scheme);
                if (result != null && result.Succeeded)
                {
                    newPrincipal = SecurityHelper.MergeUserPrincipal(newPrincipal, result.Principal);
                }
            }

            if (newPrincipal != null)
            {
                context.User = newPrincipal;
                return AuthenticateResult.Success(new AuthenticationTicket(newPrincipal, string.Join(";", policy.AuthenticationSchemes)));
            }
            else
            {
                context.User = new ClaimsPrincipal(new ClaimsIdentity());
                return AuthenticateResult.NoResult();
            }
        }

        return (context.User?.Identity?.IsAuthenticated ?? false) 
            ? AuthenticateResult.Success(new AuthenticationTicket(context.User, "context.User"))
            : AuthenticateResult.NoResult();
    }
}

在《上一章》中，咱們知道在AuthorizationPolicy中有AuthenticationSchemes和IAuthorizationRequirement兩個屬性，並詳細介紹介紹了Requirement，可是沒有提到AuthenticationSchemes的調用。

那麼，看到這裏，也就大概明白了，它與Requirements的執行是徹底獨立的，並在它以前執行，用於重置Claims，那麼爲何要重置呢？

在認證的章節介紹過，在認證階段，只會執行默認的認證Scheme，context.User就是使用context.AuthenticateAsync(DefaultAuthenticateScheme)來賦值的，當咱們但願使用非默認的Scheme，或者是想合併多個認證Scheme的Claims時，就須要使用基於Scheme的受權來重置Claims了。

它的實現也很簡單，直接使用咱們在受權策略中指定的Schemes來依次調用認證服務的AuthenticateAsync方法，並將生成的Claims合併，最後返回咱們熟悉的AuthenticateResult認證結果。

AuthorizeAsync(Requirements)

接下來再看一下PolicyEvaluator的AuthorizeAsync方法：

public class PolicyEvaluator : IPolicyEvaluator
{
    private readonly IAuthorizationService _authorization;
    public PolicyEvaluator(IAuthorizationService authorization)
    {
        _authorization = authorization;
    }

    public virtual async Task<PolicyAuthorizationResult> AuthorizeAsync(AuthorizationPolicy policy, AuthenticateResult authenticationResult, HttpContext context, object resource)
    {
        var result = await _authorization.AuthorizeAsync(context.User, resource, policy);
        if (result.Succeeded) return PolicyAuthorizationResult.Success();
        return (authenticationResult.Succeeded) ? PolicyAuthorizationResult.Forbid() : PolicyAuthorizationResult.Challenge();
    }
}

該方法會根據Requirements來完成受權，具體的實現是經過調用IAuthorizationService來實現的。

最終返回的是一個PolicyAuthorizationResult對象，並在受權失敗時，根據認證結果來返回Forbid(未受權)或Challenge(未登陸)。

public class PolicyAuthorizationResult
{
    private PolicyAuthorizationResult() { }
    public bool Challenged { get; private set; }
    public bool Forbidden { get; private set; }
    public bool Succeeded { get; private set; }
}

IAuthorizationService

而後就到了受權的核心對象AuthorizationService，也能夠稱爲受權的外交官，咱們也能夠直接在應用代碼中調用該對象來實現受權，它有以下定義：

public interface IAuthorizationService
{    
    Task<AuthorizationResult> AuthorizeAsync(ClaimsPrincipal user, object resource, string policyName);
    Task<AuthorizationResult> AuthorizeAsync(ClaimsPrincipal user, object resource, IEnumerable<IAuthorizationRequirement> requirements);
}

在AuthorizeAsync中還涉及到一個resource對象，用來實現面向資源的受權，放在《下一章》中再來介紹，而在本章與《前一章》的示例中，該值均爲null。

ASP.NET Core 中還爲IAuthorizationService提供了幾個擴展方法：

public static class AuthorizationServiceExtensions
{
    public static Task<AuthorizationResult> AuthorizeAsync(this IAuthorizationService service, ClaimsPrincipal user, string policyName) {}
    public static Task<AuthorizationResult> AuthorizeAsync(this IAuthorizationService service, ClaimsPrincipal user, AuthorizationPolicy policy) {}
    public static Task<AuthorizationResult> AuthorizeAsync(this IAuthorizationService service, ClaimsPrincipal user, object resource, IAuthorizationRequirement requirement) {}
    public static Task<AuthorizationResult> AuthorizeAsync(this IAuthorizationService service, ClaimsPrincipal user, object resource, AuthorizationPolicy policy) {}
}

其默認實現爲DefaultAuthorizationService:

public class DefaultAuthorizationService : IAuthorizationService
{
    private readonly AuthorizationOptions _options;
    private readonly IAuthorizationHandlerContextFactory _contextFactory;
    private readonly IAuthorizationHandlerProvider _handlers;
    private readonly IAuthorizationEvaluator _evaluator;
    private readonly IAuthorizationPolicyProvider _policyProvider;

    public async Task<AuthorizationResult> AuthorizeAsync(ClaimsPrincipal user, object resource, string policyName)
    {        
        var policy = await _policyProvider.GetPolicyAsync(policyName);
        return await this.AuthorizeAsync(user, resource, policy);
    }

    public async Task<AuthorizationResult> AuthorizeAsync(ClaimsPrincipal user, object resource, IEnumerable<IAuthorizationRequirement> requirements)
    {
        var authContext = _contextFactory.CreateContext(requirements, user, resource);
        var handlers = await _handlers.GetHandlersAsync(authContext);
        foreach (var handler in handlers)
        {
            await handler.HandleAsync(authContext);
            if (!_options.InvokeHandlersAfterFailure && authContext.HasFailed)
            {
                break;
            }
        }
        return _evaluator.Evaluate(authContext);
    }
}

經過上面代碼能夠看出，在《上一章》中介紹的受權策略，在這裏獲取到它的Requirements，後續便再也不須要了。而在AuthorizationService中是經過調用四大核心對象來完成受權，咱們一一來看。

IAuthorizationPolicyProvider

因爲在[Authorize]中，咱們指定的是策略的名稱，所以須要使用IAuthorizationPolicyProvider來根據名稱獲取到策略對象，默認實現爲DefaultAuthorizationPolicyProvider：

public class DefaultAuthorizationPolicyProvider : IAuthorizationPolicyProvider
{
    private readonly AuthorizationOptions _options;

    public Task<AuthorizationPolicy> GetDefaultPolicyAsync()
    {
        return Task.FromResult(_options.DefaultPolicy);
    }

    public virtual Task<AuthorizationPolicy> GetPolicyAsync(string policyName)
    {
        return Task.FromResult(_options.GetPolicy(policyName));
    }
}

在上一章中介紹過，咱們定義的策略都保存在《AuthorizationOptions》的字典中，所以在這裏只是簡單的將AuthorizationOptions中的同名方法異步化。

IAuthorizationHandlerContextFactory

受權上下文是咱們接觸較多的對象，當咱們自定義受權Handler時就會用到它，它是使用簡單工廠模式來建立的：

public class DefaultAuthorizationHandlerContextFactory : IAuthorizationHandlerContextFactory
{
    public virtual AuthorizationHandlerContext CreateContext(IEnumerable<IAuthorizationRequirement> requirements, ClaimsPrincipal user, object resource)
    {
        return new AuthorizationHandlerContext(requirements, user, resource);
    }
}

受權上下文中主要包含用戶的Claims和受權策略的Requirements：

public class AuthorizationHandlerContext
{
    private HashSet<IAuthorizationRequirement> _pendingRequirements;
    private bool _failCalled;
    private bool _succeedCalled;

    public AuthorizationHandlerContext(IEnumerable<IAuthorizationRequirement> requirements, ClaimsPrincipal user, object resource)
    {
        Requirements = requirements; User = user; Resource = resource;
        _pendingRequirements = new HashSet<IAuthorizationRequirement>(requirements);
    }

    public virtual bool HasFailed { get { return _failCalled; } }
    public virtual bool HasSucceeded => !_failCalled && _succeedCalled && !_pendingRequirements.Any();
    public virtual void Fail()
    {
        _failCalled = true;
    }
    public virtual void Succeed(IAuthorizationRequirement requirement)
    {
        _succeedCalled = true;
        _pendingRequirements.Remove(requirement);
    }
}

如上，_pendingRequirements中保存着全部待驗證的Requirements，驗證成功的Requirement則從中移除。

IAuthorizationHandlerProvider

兜兜轉轉，終於進入到了受權的最終驗證邏輯中了，首先，使用IAuthorizationHandlerProvider來獲取到全部的受權Handler。

IAuthorizationHandlerProvider的默認實現爲DefaultAuthorizationHandlerProvider:

public class DefaultAuthorizationHandlerProvider : IAuthorizationHandlerProvider
{
    private readonly IEnumerable<IAuthorizationHandler> _handlers;

    public DefaultAuthorizationHandlerProvider(IEnumerable<IAuthorizationHandler> handlers)
    {
        _handlers = handlers;
    }

    public Task<IEnumerable<IAuthorizationHandler>> GetHandlersAsync(AuthorizationHandlerContext context)
        => Task.FromResult(_handlers);
}

在《上一章》中，咱們還介紹到，咱們定義的Requirement，能夠直接實現IAuthorizationHandler接口，也能夠單獨定義Handler，可是須要註冊到DI系統中去。

在默認的AuthorizationHandlerProvider中，會從DI系統中獲取到咱們註冊的全部Handler，最終調用其HandleAsync方法。

咱們在實現IAuthorizationHandler接口時，一般是繼承自AuthorizationHandler<TRequirement>來實現，它有以下定義：

public abstract class AuthorizationHandler<TRequirement> : IAuthorizationHandler where TRequirement : IAuthorizationRequirement
{
    public virtual async Task HandleAsync(AuthorizationHandlerContext context)
    {
        foreach (var req in context.Requirements.OfType<TRequirement>())
        {
            await HandleRequirementAsync(context, req);
        }
    }

    protected abstract Task HandleRequirementAsync(AuthorizationHandlerContext context, TRequirement requirement);
}

如上，首先會在HandleAsync過濾出與Requirement對匹配的Handler，而後再調用其HandleRequirementAsync方法。

那咱們定義的直接實現IAuthorizationHandler了接口的Requirement又是如何執行的呢？

在AddAuthorization擴展方法中能夠看到，默認還爲IAuthorizationHandler註冊了一個PassThroughAuthorizationHandler，定義以下：

public class PassThroughAuthorizationHandler : IAuthorizationHandler
{
    public async Task HandleAsync(AuthorizationHandlerContext context)
    {
        foreach (var handler in context.Requirements.OfType<IAuthorizationHandler>())
        {
            await handler.HandleAsync(context);
        }
    }
}

它負責調用該策略中全部實現了IAuthorizationHandler接口的Requirement。

IAuthorizationEvaluator

最後，經過調用IAuthorizationEvaluator接口，來完成最終的受權結果，默認實現爲DefaultAuthorizationEvaluator:

public class DefaultAuthorizationEvaluator : IAuthorizationEvaluator
{
    public AuthorizationResult Evaluate(AuthorizationHandlerContext context)
        => context.HasSucceeded
            ? AuthorizationResult.Success()
            : AuthorizationResult.Failed(context.HasFailed
                ? AuthorizationFailure.ExplicitFail()
                : AuthorizationFailure.Failed(context.PendingRequirements));
}

當咱們在一個策略中指定多個Requirement時，只有所有驗證經過時，受權上下文中的HasSucceeded纔會爲True，而HasFailed表明受權結果的顯式失敗。

這裏根據受權上下文的驗證結果來生成受權結果：

public class AuthorizationResult
{
    public bool Succeeded { get; private set; }
    public AuthorizationFailure Failure { get; private set; }
    public static AuthorizationResult Success() => new AuthorizationResult { Succeeded = true };
    public static AuthorizationResult Failed(AuthorizationFailure failure) => new AuthorizationResult { Failure = failure };
    public static AuthorizationResult Failed() => new AuthorizationResult { Failure = AuthorizationFailure.ExplicitFail() };
}

public class AuthorizationFailure
{
    private AuthorizationFailure() { }
    public bool FailCalled { get; private set; }
    public IEnumerable<IAuthorizationRequirement> FailedRequirements { get; private set; }
    public static AuthorizationFailure ExplicitFail()
    {
        return new AuthorizationFailure { FailCalled = true, FailedRequirements = new IAuthorizationRequirement[0] };
    }
    public static AuthorizationFailure Failed(IEnumerable<IAuthorizationRequirement> failed)
        => new AuthorizationFailure { FailedRequirements = failed };

}

整個受權流程的結構大體以下：

總結

經過對 ASP.NET Core 受權系統執行流程的探索，能夠看出受權是主要是經過調用IAuthorizationService來完成的，而受權策略的本質是提供 Requirement ，咱們徹底可使用它們兩個來完成各類靈活的受權方式，而不用侷限於策略。在 ASP.NET Core 中，還提供了基於資源的受權，放在《下一章》中來介紹，並會簡單演示一下在一個通用權限管理系統中如何來受權。