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ASP.NET Core 认证与授权[6]:授权策略是怎么执行的?

在上一章中,详细介绍了 ASP.NET Core 中的授权策略,在需要授权时,只需要在对应的Controler或者Action上面打上[Authorize]特性,并指定要执行的策略名称即可,但是,授权策略是怎么执行的呢?怀着一颗好奇的心,忍不住来探索一下它的执行流程。

目录

  1. MVC中的授权
  2. IPolicyEvaluator
  3. IAuthorizationService

在《(上一章》中提到,AuthorizeAttribute只是一个简单的实现了IAuthorizeData接口的特性,并且在 ASP.NET Core 授权系统中并没有使用到它。我们知道在认证中,还有一个UseAuthentication扩展方法来激活认证系统,但是在授权中并没有类似的机制。

这是因为当我们使用[Authorize]通常是在MVC中,由MVC来负责激活授权系统。本来在这个系列的文章中,我并不想涉及到MVC的知识,但是为了能更好的理解授权系统的执行,就来简单介绍一下MVC中与授权相关的知识。

MVC中的授权

当我们使用MVC时,首先会调用MVC的AddMvc扩展方法,用来注册一些MVC相关的服务:

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中的前三个方法都属于 ASP.NET Core 《Security》项目中提供的扩展方法,其中前两个在前面几章已经介绍过了,对于AddAuthorizationPolicyEvaluator放到后面再来介绍,我们先来看一下MVC中的AuthorizationApplicationModelProvider

AuthorizationApplicationModelProvider

在MVC中有一个ApplicationModel的概念,它用来封装Controller, Filter, ApiExplorer等。对应的,在MVC中还提供了一系列的ApplicationModelProvider来初始化ApplicationModel的各个部分,而AuthorizationApplicationModelProvider就是用来初始化与授权相关的部分。

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,首先查找每个Controller中实现了IAuthorizeData接口的特性,然后将其转化为AuthorizeFilter并添加到Controller的Filter集合中,紧接着再查找实现了IAllowAnonymous接口的特性,将其转化为AllowAnonymousFilter过滤器也添加到Filter集合中,然后以同样的逻辑查找Action上的特性并添加到Action的Filter集合中。

其中的关键点就是将IAuthorizeData(也就是通过我们熟悉的[Authorize]特性)转化为MVC中的AuthorizeFilter过滤器:

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》中已经介绍过了,我们往下看看AuthorizeFilter的实现。

AuthorizeFilter

在MVC中有一个AuthorizeFilter过滤器,类似我们在ASP.NET 4.x中所熟悉的[Authorize],它实现了IAsyncAuthorizationFilter接口,定义如下:

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);

        ... // 如果授权失败,返回ChallengeResult或ForbidResult
    }
}

AuthorizeFilter的OnAuthorizationAsync方法会在Action执行之前触发,其调用IPolicyEvaluator来完成授权,将执行流程切回到 ASP.NET Core 授权系统中。关于MVC中IApplicationModelProvider以及Filter的概念,在以后MVC系列的文章中再来详细介绍,下面就继续介绍 ASP.NET Core 的授权系统,也就是《Security》项目。

IPolicyEvaluator

IPolicyEvaluator是MVC调用授权系统的入口点,其定义如下:

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

在上面介绍的AddMVC中,调用了AddAuthorizationPolicyEvaluator扩展方法,它有如下定义:

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

由此可知IPolicyEvaluator的默认实现为PolicyEvaluator,我们就从它入手,来一步一步解剖 ASP.NET Core 授权系统的执行步骤。

AuthorizeFilter中,依次调到了AuthenticateAsyncAuthorizeAsync方法,我们就一一来看。

AuthenticateAsync(AuthenticationSchemes)

为什么还有一个AuthenticateAsync方法呢,这不是在认证阶段执行的吗?我们看下它的实现:

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,我们知道在AuthorizationPolicy中有AuthenticationSchemesIAuthorizationRequirement两个属性,并详细介绍介绍了Requirement,但是没有提到AuthenticationSchemes的调用。

那么,看到这里,也就大概明白了,它与Requirements的执行是完全独立的,并在它之前执行,用于重置Claims,那么为什么要重置呢?

在认证的章节介绍过,在认证阶段,只会执行默认的认证Scheme,context.User就是使用context.AuthenticateAsync(DefaultAuthenticateScheme)来赋值的,当我们希望使用非默认的Scheme,或者是想合并多个认证Scheme的Claims时,就需要使用基于Scheme的授权来重置Claims了。

它的实现也很简单,直接使用我们在授权策略中指定的Schemes来依次调用认证服务的AuthenticateAsync方法,并将生成的Claims合并,最后返回我们熟悉的AuthenticateResult认证结果。

AuthorizeAsync(Requirements)

接下来再看一下PolicyEvaluatorAuthorizeAsync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();
    }
}

该方法会根据Requirements来完成授权,具体的实现是通过调用IAuthorizationService来实现的。

最终返回的是一个PolicyAuthorizationResult对象,并在授权失败时,根据认证结果来返回Forbid(未授权)Challenge(未登录)

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

IAuthorizationService

然后就到了授权的核心对象AuthorizationService,也可以称为授权的外交官,我们也可以直接在应用代码中调用该对象来实现授权,它有如下定义:

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

AuthorizeAsync中还涉及到一个resource对象,用来实现面向资源的授权,放在《下一章》中再来介绍,而在本章与《前一章》的示例中,该值均为null

ASP.NET Core 中还为IAuthorizationService提供了几个扩展方法:

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) {}
}

其默认实现为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);
    }
}

通过上面代码可以看出,在《上一章》中介绍的授权策略,在这里获取到它的Requirements,后续便不再需要了。而在AuthorizationService中是通过调用四大核心对象来完成授权,我们一一来看。

IAuthorizationPolicyProvider

由于在[Authorize]中,我们指定的是策略的名称,因此需要使用IAuthorizationPolicyProvider来根据名称获取到策略对象,默认实现为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));
    }
}

在上一章中介绍过,我们定义的策略都保存在《AuthorizationOptions》的字典中,因此在这里只是简单的将AuthorizationOptions中的同名方法异步化。

IAuthorizationHandlerContextFactory

授权上下文是我们接触较多的对象,当我们自定义授权Handler时就会用到它,它是使用简单工厂模式来创建的:

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

授权上下文中主要包含用户的Claims和授权策略的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中保存着所有待验证的Requirements,验证成功的Requirement则从中移除。

IAuthorizationHandlerProvider

兜兜转转,终于进入到了授权的最终验证逻辑中了,首先,使用IAuthorizationHandlerProvider来获取到所有的授权Handler

IAuthorizationHandlerProvider的默认实现为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,我们还介绍到,我们定义的Requirement,可以直接实现IAuthorizationHandler接口,也可以单独定义Handler,但是需要注册到DI系统中去。

在默认的AuthorizationHandlerProvider中,会从DI系统中获取到我们注册的所有Handler,最终调用其HandleAsync方法。

我们在实现IAuthorizationHandler接口时,通常是继承自AuthorizationHandler<TRequirement>来实现,它有如下定义:

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,首先会在HandleAsync过滤出与Requirement对匹配的Handler,然后再调用其HandleRequirementAsync方法。

那我们定义的直接实现IAuthorizationHandler了接口的Requirement又是如何执行的呢?

AddAuthorization扩展方法中可以看到,默认还为IAuthorizationHandler注册了一个PassThroughAuthorizationHandler,定义如下:

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

它负责调用该策略中所有实现了IAuthorizationHandler接口的Requirement

IAuthorizationEvaluator

最后,通过调用IAuthorizationEvaluator接口,来完成最终的授权结果,默认实现为DefaultAuthorizationEvaluator:

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

当我们在一个策略中指定多个Requirement时,只有全部验证通过时,授权上下文中的HasSucceeded才会为True,而HasFailed代表授权结果的显式失败。

这里根据授权上下文的验证结果来生成授权结果:

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 };

}

整个授权流程的结构大致如下:

authorization_service

总结

通过对 ASP.NET Core 授权系统执行流程的探索,可以看出授权是主要是通过调用IAuthorizationService来完成的,而授权策略的本质是提供 Requirement ,我们完全可以使用它们两个来完成各种灵活的授权方式,而不用局限于策略。在 ASP.NET Core 中,还提供了基于资源的授权,放在《下一章》中来介绍,并会简单演示一下在一个通用权限管理系统中如何来授权。

posted @ 2017-11-22 08:40 雨夜朦胧 阅读(...) 评论(...) 编辑 收藏