This post details a local privilege escalation (LPE) vulnerability I found in Dell’s SupportAssist[0] tool. The bug is in a kernel driver loaded by the tool, and is pretty similar to bugs found by ReWolf in ntiolib.sys/winio.sys[1], and those found by others in ASMMAP/ASMMAP64[2]. These bugs are pretty interesting because they can be used to bypass driver signature enforcement (DSE) ad infinitum, or at least until they’re no longer compatible with newer operating systems.

Dell’s SupportAssist is, according to the site, “(..) now preinstalled on most of all new Dell devices running Windows operating system (..)”. It’s primary purpose is to troubleshoot issues and provide support capabilities both to the user and to Dell. There’s quite a lot of functionality in this software itself, which I spent quite a bit of time reversing and may blog about at a later date.

Bug

Calling this a “bug” is really a misnomer; the driver exposes this functionality eagerly. It actually exposes a lot of functionality, much like some of the previously mentioned drivers. It provides capabilities for reading and writing the model-specific register (MSR), resetting the 1394 bus, and reading/writing CMOS.

The driver is first loaded when the SupportAssist tool is launched, and the filename is pcdsrvc_x64.pkms on x64 and pcdsrvc.pkms on x86. Incidentally, this driver isn’t actually even built by Dell, but rather another company, PC-Doctor[3]. This company provides “system health solutions” to a variety of companies, including Dell, Intel, Yokogawa, IBM, and others. Therefore, it’s highly likely that this driver can be found in a variety of other products…

Once the driver is loaded, it exposes a symlink to the device at PCDSRVC{3B54B31B-D06B6431-06020200}_0 which is writable by unprivileged users on the system. This allows us to trigger one of the many IOCTLs exposed by the driver; approximately 30. I found a DLL used by the userland agent that served as an interface to the kernel driver and conveniently had symbol names available, allowing me to extract the following:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 // 0x222004 = driver activation ioctl // 0x222314 = IoDriver::writePortData // 0x22230c = IoDriver::writePortData // 0x222304 = IoDriver::writePortData // 0x222300 = IoDriver::readPortData // 0x222308 = IoDriver::readPortData // 0x222310 = IoDriver::readPortData // 0x222700 = EcDriver::readData // 0x222704 = EcDriver::writeData // 0x222080 = MemDriver::getPhysicalAddress // 0x222084 = MemDriver::readPhysicalMemory // 0x222088 = MemDriver::writePhysicalMemory // 0x222180 = Msr::readMsr // 0x222184 = Msr::writeMsr // 0x222104 = PciDriver::readConfigSpace // 0x222108 = PciDriver::writeConfigSpace // 0x222110 = PciDriver::? // 0x22210c = PciDriver::? // 0x222380 = Port1394::doesControllerExist // 0x222384 = Port1394::getControllerConfigRom // 0x22238c = Port1394::getGenerationCount // 0x222388 = Port1394::forceBusReset // 0x222680 = SmbusDriver::genericRead // 0x222318 = SystemDriver::readCmos8 // 0x22231c = SystemDriver::writeCmos8 // 0x222600 = SystemDriver::getDevicePdo // 0x222604 = SystemDriver::getIntelFreqClockCounts // 0x222608 = SystemDriver::getAcpiThermalZoneInfo

Immediately the MemDriver class jumps out. After some reversing, it appeared that these functions do exactly as expected: allow userland services to both read and write arbitrary physical addresses. There are a few quirks, however.

To start, the driver must first be “unlocked” in order for it to begin processing control codes. It’s unclear to me if this is some sort of hacky event trigger or whether the kernel developers truly believed this would inhibit malicious access. Either way, it’s goofy. To unlock the driver, a simple ioctl with the proper code must be sent. Once received, the driver will process control codes for the lifetime of the system.

To unlock the driver, we just execute the following:

1 2 3 4 5 6 7 8 BOOL bResult; DWORD dwRet; SIZE_T code = 0xA1B2C3D4, outBuf; bResult = DeviceIoControl(hDriver, 0x222004, &code, sizeof(SIZE_T), &outBuf, sizeof(SIZE_T), &dwRet, NULL);

Once the driver receives this control code and validates the received code (0xA1B2C3D4), it sets a global flag and begins accepting all other control codes.

Exploitation

From here, we could exploit this the same way rewolf did [4]: read out physical memory looking for process pool tags, then traverse these until we identify our process as well as a SYSTEM process, then steal the token. However, PCD appears to give us a shortcut via getPhysicalAddress ioctl. If this does indeed return the physical address of a given virtual address (VA), we can simply find the physical of our VA and enable a couple token privileges[5] using the writePhysicalMemory ioctl.

Here’s how the getPhysicalAddress function works:

1 2 3 4 5 6 7 8 v5 = IoAllocateMdl(**(PVOID **)(a1 + 0x18), 1u, 0, 0, 0i64); v6 = v5; if ( !v5 ) return 0xC0000001i64; MmProbeAndLockPages(v5, 1, 0); **(_QWORD **)(v3 + 0x18) = v4 & 0xFFF | ((_QWORD)v6[1].Next << 0xC); MmUnlockPages(v6); IoFreeMdl(v6);

Keen observers will spot the problem here; the MmProbeAndLockPages call is passing in UserMode for the KPROCESSOR_MODE, meaning we won’t be able to resolve any kernel mode VAs, only usermode addresses.

We can still read chunks of physical memory unabated, however, as the readPhysicalMemory function is quite simple:

1 2 3 4 5 if ( !DoWrite ) { memmove(a1, a2, a3); return 1; }

They reuse a single function for reading and writing physical memory; we’ll return to that. I decided to take a different approach than rewolf for a number of reasons with great results.

Instead, I wanted to toggle on SeDebugPrivilege for my current process token. This would require finding the token in memory and writing a few bytes at a field offset. To do this, I used readPhysicalMemory to read chunks of memory of size 0x10000000 and checked for the first field in a _TOKEN, TokenSource. In a user token, this will be the string User32 . Once we’ve identified this, we double check that we’ve found a token by validating the TokenLuid, which we can obtain from userland using the GetTokenInformation API.

In order to speed up the memory search, I only iterate over the addresses that match the token’s virtual address byte index. Essentially, when you convert a virtual address to a physical address (PA) the byte index, or the lower 12 bits, do not change. To demonstrate, assume we have a VA of 0xfffff8a001cc2060. Translating this to a physical address then:

1 2 3 4 5 6 7 8 kd> !pte fffff8a001cc2060 VA fffff8a001cc2060 PXE at FFFFF6FB7DBEDF88 PPE at FFFFF6FB7DBF1400 PDE at FFFFF6FB7E280070 PTE at FFFFF6FC5000E610 contains 000000007AC84863 contains 00000000030D4863 contains 0000000073147863 contains E6500000716FD963 pfn 7ac84 ---DA--KWEV pfn 30d4 ---DA--KWEV pfn 73147 ---DA--KWEV pfn 716fd -G-DA--KW-V kd> ? 716fd * 0x1000 + 060 Evaluate expression: 1903153248 = 00000000`716fd060

So our physical address is 0x716fd060 (if you’d like to read more about converting VA to PA, check out this great Microsoft article[6]). Notice the lower 12 bits remain the same between VA/PA. The search loop then boiled down to the following code:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 uStartAddr = uStartAddr + (VirtualAddress & 0xfff); for (USHORT chunk = 0; chunk < 0xb; ++chunk) { lpMemBuf = ReadBlockMem(hDriver, uStartAddr, 0x10000000); for(SIZE_T i = 0; i < 0x10000000; i += 0x1000, uStartAddr += 0x1000){ if (memcmp((DWORD)lpMemBuf + i, "User32 ", 8) == 0){ if (TokenId <= 0x0) FetchTokenId(); if (*(DWORD*)((char*)lpMemBuf + i + 0x10) == TokenId) { hTokenAddr = uStartAddr; break; } } } HeapFree(GetProcessHeap(), 0, lpMemBuf); if (hTokenAddr > 0x0) break; }

Once we identify the PA of our token, we trigger two separate writes at offset 0x40 and offset 0x48, or the Enabled and Default fields of a _TOKEN. This sometimes requires a few runs to get right (due to mapping, which I was too lazy to work out), but is very stable.

You can find the source code for the bug here.

Timeline

04/05/18 – Vulnerability reported

04/06/18 – Initial response from Dell

04/10/18 – Status update from Dell

04/18/18 – Status update from Dell

05/16/18 – Patched version released (v2.2)

References

[0] http://www.dell.com/support/contents/us/en/04/article/product-support/self-support-knowledgebase/software-and-downloads/supportassist [1] http://blog.rewolf.pl/blog/?p=1630 [2] https://www.exploit-db.com/exploits/39785/ [3] http://www.pc-doctor.com/ [4] https://github.com/rwfpl/rewolf-msi-exploit [5] https://github.com/hatRiot/token-priv [6] https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/converting-virtual-addresses-to-physical-addresses\