Windows内核扩展机制研究

2019-03-26 54258人围观 ,发现 1 个不明物体 系统安全

近期,我自己写了一个内核模式驱动器,跟我平时写的很多代码一样,这个驱动器也存在Bug。 确切来说,这个驱动器可以防止其他驱动器正常加载,并导致系统崩溃。

实际上,很多启动器会默认它们的初始化进程(DriverEntry)能够成功执行,因此在出现问题时它们是无法正确处理的。j00ru曾在几年前发布的博文中提过几种案例,而且其中有很多都跟目前的Windows版本有关,而我只对其中的一个驱动器比较感兴趣。这里最吸引我的还是“Windows内核主机扩展机制”,本文的主要内容也是关于它的。

这个驱动器是Bam.sys(Background Activity Moderator)- Windows 10 v1709(RS3)中新引入的一种驱动器。当它的DriverEntry中途出现问题时,调用栈会导致系统发生崩溃:

从崩溃信息中我们可以看到,Bam.sys注册了一个进程创建回调,并且在卸载之前忘记注销它了。接下来,当一个进程被创建/终止时,系统会尝试调用这个回调,并引用旧指针,然后导致系统崩溃。

有趣的并不是系统发生崩溃,而是Bam.sys到底是如何注册这个回调的。一般来说,进程创建回调是通过nt!PsSetCreateProcessNotifyRoutine(Ex)注册的,它会向nt!PspCreateProcessNotifyRoutine数组中添加这个回调。接下来,当进程被创建/终止时,nt!PspCallProcessNotifyRoutines会迭代循环这个数组,并调用所有已注册的回调。然而,如果我们在WinDbg中运行“!wdbgark.wa_systemcb /typeprocess“的话,我们将会看到这个数组中并不存在Bam.sys所使用的回调:

原来,Bam.sys使用了另一种机制来注册它的回调:

4.png

5.png

查看nt!PspCallProcessNotifyRoutines后我们发现,其中有一个指向nt!PspBamExtensionHost的显示引用。它会使用这种“扩展主机”来检索所谓的“扩展表”,并调用扩展表中的第一个函数,即bam!BampCreateProcessCallback。

在IDA中打开Bam.sys,我们可以直接看到bam!BampCreateProcessCallback,并搜索它的xrefs。幸好它只有一个,就在bam!BampRegisterKernelExtension中:

我怀疑Bam!BampCreateProcessCallback并没有通过常规的回调注册机制来进行注册。实际上,它被存在了一个名叫Bam!BampKernelCalloutTable的函数表中,而这个表会和其他参数一起传递给nt!ExRegisterExtension函数(文档中未记载)。

唯一一个我找到的跟这个函数相关的就是ntosifs.h头文件,其中包含了nt!ExRegisterExtension函数的原型以及_EX_EXTENSION_REGISTRATION_1结构体。

nt!ExRegisterExtension函数原型和_EX_EXTENSION_REGISTRATION_1:

NTKERNELAPINTSTATUS ExRegisterExtension (

    _Outptr_ PEX_EXTENSION *Extension,

    _In_ ULONG RegistrationVersion,

    _In_ PVOID RegistrationInfo

);

 

typedefstruct _EX_EXTENSION_REGISTRATION_1 {

    USHORT ExtensionId;

    USHORT ExtensionVersion;

    USHORT FunctionCount;

    VOID *FunctionTable;

    PVOID *HostInterface;

    PVOID DriverObject;

}EX_EXTENSION_REGISTRATION_1, *PEX_EXTENSION_REGISTRATION_1;

注意:nt!ExRegisterExtension的伪代码可在附录B中找到。

我们可以看到,nt!ExRegisterExtension会搜索名称匹配RegistrationInfo的主机。那么问题来了,这些主机是从哪儿来的呢?

在初始化的过程中,NTOS会多次调用nt!ExRegisterHost。每次调用时,它都会传递一个结构体来识别列表中预定义的驱动器(完整列表可在附录A中找到)。比如说,Bam.sys会使用下列调用来初始化一台主机:

7.png

nt!ExRegisterHost会分配一个type_HOST_LIST_ENTRY结构体,并使用调用方提供的参数来进行初始化,然后将其添加到nt!ExpHostList结尾。_HOST_LIST_ENTRY结构体文档未记载,它的大致结构如下:

struct_HOST_LIST_ENTRY

{

    _LIST_ENTRY List;

    DWORD RefCount;

    USHORT ExtensionId;

    USHORT ExtensionVersion;

    USHORT FunctionCount; // number ofcallbacks that the extension

                          // contains

    POOL_TYPE PoolType;   // where this host is allocated

    PVOID HostInterface; // table of unexportednt functions,

                         // to be used by thedriver to which

                         // this extensionbelongs

    PVOID FunctionAddress; // optional, rarelyused.

                           // This callback iscalled before

                           // and after anextension for this

                           // host isregistered / unregistered

    PVOID ArgForFunction; // will be sent tothe function saved here

    _EX_RUNDOWN_REF RundownRef;

    _EX_PUSH_LOCK Lock;

    PVOID FunctionTable; // a table of thecallbacks that the

                         // driver “registers”

    DWORD Flags;         // Only uses one bit.

                         // Not sure about itsmeaning.

}HOST_LIST_ENTRY, *PHOST_LIST_ENTRY;

当其中一个预定义的驱动器加载时,它会使用nt!ExRegisterExtension注册一个扩展,并提供一个RegistrationInfo结构体,其中包括一个函数表。这个函数表会被置入匹配主机的FunctionTable成员中。这些函数随后会被NTOS在特定的场景下调用,这也就是所谓的回调了。

之前我们看到了nt!ExRegisterExtension的部分功能会被设置为RegistrationInfo->HostInterface(包含一个全局变量),并指向主机结构体中的某些数据。

每一个注册了扩展的驱动器都有一个NTOS设置的初始化主机,这种主机包含一个HostInterface,它指向HTOS函数预定一个一个表,不同的驱动器会收到不同的HostInterface,但有些则不会收到。

比如说,Bam.sys收到的就是下面这个HostInterface:

实际上,这里所谓的“内核扩展机制”即使一种双向通信端口:驱动器提供一个“回调”列表,以供在不同场景下调用,然后在各自内部使用场景中接收不同的函数集。

下面给出的是Bam.sys的回调样本:

BampCreateProcessCallback

BampSetThrottleStateCallback

BampGetThrottleStateCallback

BampSetUserSettings

BampGetUserSettingsHandle

负责Bam.sys初始化的主机预先“知道”它需要接收包含上面这5个函数的函数表。这些函数必须按照上述顺序提供,因此这里是通过索引来调用的。我们可以看到,nt!PspBamExtensionHost->FunctionTable[4]调用情况:

总而言之,Windows中存在一种机制,可以用来“扩展”NTOS,具体过程是先注册某些回调函数,然后接收驱动可以使用的未导出函数。我并不清楚这个知识点是否能发挥实际作用,但觉得这方面内容比较有趣,值得与大家分享。希望可以给大家提供帮助,感谢阅读!

附录A-NTOS初始化的扩展主机

10.png

附录B-函数伪代码

ExRegisterExtension.c:

NTSTATUSExRegisterExtension(_Outptr_ PEX_EXTENSION *Extension, _In_ ULONGRegistrationVersion, _In_ PREGISTRATION_INFO RegistrationInfo)

{

       // Validate that version is ok and thatFunctionTable is not sent without FunctionCount or vise-versa.

       if ( (RegistrationVersion &0xFFFF0000 != 0x10000) || (RegistrationInfo->FunctionTable == nullptr&& RegistrationInfo->FunctionCount != 0) )

       {

              return STATUS_INVALID_PARAMETER;

       }

 

       // Skipping over some lock-related stuff,

      

       // Find the host with the matchingversion and id.

       PHOST_LIST_ENTRY pHostListEntry;

       pHostListEntry =ExpFindHost(RegistrationInfo->ExtensionId,RegistrationInfo->ExtensionVersion);

 

       // More lock-related stuff.     

 

       if (!pHostListEntry)

       {

              return STATUS_NOT_FOUND;

       }

 

       // Verify that the FunctionCount in thehost doesn't exceed the FunctionCount supplied by the caller.

       if (RegistrationInfo->FunctionCount< pHostListEntry->FunctionCount)

       {

       ExpDereferenceHost(pHostListEntry);

              return STATUS_INVALID_PARAMETER;

       }

 

       // Check that the number of functions inFunctionTable matches the amount in FunctionCount.

       PVOID FunctionTable =RegistrationInfo->FunctionTable;

       for (int i = 0; i <RegistrationInfo->FunctionCount; i++)

       {     

              if (RegistrationInfo->FunctionTable[i] == nullptr )

              {

              ExpDereferenceHost(pHostListEntry);

                     returnSTATUS_ACCESS_DENIED;

              }

       }

 

       // skipping over some more lock-relatedstuff

 

       // Check if there is already an extensionregistered for this host.

       if (pHostListEntry->FunctionTable !=nullptr || FlagOn(pHostListEntry->Flags, 1) )

       {

              // There is something related tolocks here

       ExpDereferenceHost(pHostListEntry);

              returnSTATUS_OBJECT_NAME_COLLISION;

       }

 

       // If there is a callback function forthis host, call it before registering the extension, with 0 as the firstparameter.

       if (pHostListEntry->FunctionAddress)

       {

       pHostListEntry->FunctionAddress(0,pHostListEntry->ArgForFunction);

       }

 

       // Set the FunctionTable in the host tothe table supplied by the caller, or to MmBadPointer if a table wasn'tsupplied.

       if (RegistrationInfo->FunctionTable ==nullptr)

       {

              pHostListEntry->FunctionTable= nt!MmBadPointer;

       }

       else

       {

              pHostListEntry->FunctionTable= RegistrationInfo->FunctionTable;

       }

 

       pHostListEntry->RundownRef = 0;

 

       // If there is a callback function forthis host, call it after registering the extension, with 1 as the firstparameter.

       if (pHostListEntry->FunctionAddress)

       {

       pHostListEntry->FunctionAddress(1,pHostListEntry->ArgForFunction);

       }

 

       // Here there is some more lock-relatedstuff

 

       // Set the HostTable of the callingdriver to the table of functions listed in the host.

       if (RegistrationInfo->HostTable !=nullptr)

       {

       *(PVOID)RegistrationInfo->HostTable= pHostListEntry->hostInterface;

       }

 

       // Return the initialized host to thecaller in the output Extension parameter.

       *Extension = pHostListEntry;

       return STATUS_SUCCESS;

}

ExRegisterHost.c:

NTSTATUSExRegisterHost(_Out_ PHOST_LIST_ENTRY ExtensionHost, _In_ ULONG Unused, _In_PHOST_INFORMATION HostInformation)

{

       NTSTATUS Status = STATUS_SUCCESS;

 

       // Allocate memory for a newHOST_LIST_ENTRY

       PHOST_LIST_ENTRY p =ExAllocatePoolWithTag(HostInformation->PoolType, 0x60, 'HExE');

       if (p == nullptr)

       {

              returnSTATUS_INSUFFICIENT_RESOURCES;

       }

 

       //

       // Initialize a new HOST_LIST_ENTRY

       //

       p->Flags &= 0xFE;

       p->RefCount = 1;

       p->FunctionTable = 0;

       p->ExtensionId =HostInformation->ExtensionId;

       p->ExtensionVersion =HostInformation->ExtensionVersion;

       p->hostInterface =HostInformation->hostInterface;

       p->FunctionAddress =HostInformation->FunctionAddress;

       p->ArgForFunction =HostInformation->ArgForFunction;                    

       p->Lock = 0;                   

       p->RundownRef = 0;             

 

       // Search for an existing listEntry withthe same version and id.

       PHOST_LIST_ENTRY listEntry = ExpFindHost(HostInformation->ExtensionId,HostInformation->ExtensionVersion);

       if (listEntry)

       {

              Status =STATUS_OBJECT_NAME_COLLISION;

              ExpDereferenceHost(p);

              ExpDereferenceHost(listEntry);

       }

       else

       {

              // Insert the new HOST_LIST_ENTRYto the end of ExpHostList.

              if ( *lastHostListEntry !=&firstHostListEntry )

              {

                    __fastfail();

              }

 

              firstHostListEntry->Prev =&p;

              p->Next = firstHostListEntry;

              lastHostListEntry = p;

 

              ExtensionHost = p;

       }

      

       return Status;

}

ExpFindHost.c:

PHOST_LIST_ENTRYExpFindHost(USHORT ExtensionId, USHORT ExtensionVersion)

{

       PHOST_LIST_ENTRY entry;

       for (entry == ExpHostList; ; entry =entry->Next)

       {

              if (entry == &ExpHostList)

              {

                     return 0;

              }

             

              if ( *(entry->ExtensionId) ==ExtensionId && *(entry->ExtensionVersion) == ExtensionVersion )

              {

                     break;

              }

       }

InterlockedIncrement(entry->RefCount);

       return entry;

}

ExpDereferenceHost.c:

voidExpDereferenceHost(PHOST_LIST_ENTRY Host)

{

      if (InterlockedExchangeAdd(Host.RefCount, 0xFFFFFFFF) == 1 )

       {

                  ExFreePoolWithTag(Host, 0);

       }

}

附录C-结构体定义

struct_HOST_INFORMATION

{

    USHORT ExtensionId;

    USHORT ExtensionVersion;

    DWORD FunctionCount;

    POOL_TYPE PoolType;

    PVOID HostInterface;

    PVOID FunctionAddress;

    PVOID ArgForFunction;

    PVOID unk;

}HOST_INFORMATION, *PHOST_INFORMATION;

 

struct_HOST_LIST_ENTRY

{

    _LIST_ENTRY List;

    DWORD RefCount;

    USHORT ExtensionId;

    USHORT ExtensionVersion;

    USHORT FunctionCount; // number ofcallbacks that the

                          // extension contains

    POOL_TYPE PoolType;   // where this host is allocated

    PVOID HostInterface;  // table of unexported nt functions,

                          // to be used by thedriver to which

                          // this extensionbelongs

    PVOID FunctionAddress; // optional, rarelyused.

                           // This callback iscalled before and   

                           // after anextension for this host

                           // is registered /unregistered

    PVOID ArgForFunction; // will be sent tothe function saved here

    _EX_RUNDOWN_REF RundownRef;

    _EX_PUSH_LOCK Lock;

    PVOID FunctionTable;    // a table of the callbacks that

                            // the driver“registers”

DWORDFlags;                // Only uses oneflag.

                            // Not sure about its meaning.

}HOST_LIST_ENTRY, *PHOST_LIST_ENTRY;;

 

struct_EX_EXTENSION_REGISTRATION_1

{

    USHORT ExtensionId;

    USHORT ExtensionVersion;

    USHORT FunctionCount;

    PVOID FunctionTable;

    PVOID *HostTable;

    PVOID DriverObject;

}EX_EXTENSION_REGISTRATION_1,*PEX_EXTENSION_REGISTRATION_1;

* 参考来源:medium,FB小编Alpha_h4ck编译,转载请注明来自FreeBuf.COM

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