On 8/11/2025 7:55 PM, Chao Gao wrote:
The FPU support for CET virtualization has already been merged into 6.17-rc1.
Building on that, this series introduces Intel CET virtualization support for
KVM.
Changes in v12:
1. collect Tested-by tags from John and Mathias.
2. use less verbose names for KVM rdmsr/wrmsr emulation APIs in patch 1/2
(Sean/Xin)
3. refer to s_cet, ssp, and ssp_table in a consistent order in patch 22
(Xin)
Please note that I didn't include Mathias' patch, which makes CR4.CET
guest-owned. I expect that patch to be posted separately.
---
Control-flow Enforcement Technology (CET) is a kind of CPU feature used
to prevent Return/CALL/Jump-Oriented Programming (ROP/COP/JOP) attacks.
It provides two sub-features(SHSTK,IBT) to defend against ROP/COP/JOP
style control-flow subversion attacks.
Shadow Stack (SHSTK):
A shadow stack is a second stack used exclusively for control transfer
operations. The shadow stack is separate from the data/normal stack and
can be enabled individually in user and kernel mode. When shadow stack
is enabled, CALL pushes the return address on both the data and shadow
stack. RET pops the return address from both stacks and compares them.
If the return addresses from the two stacks do not match, the processor
generates a #CP.
Indirect Branch Tracking (IBT):
IBT introduces new instruction(ENDBRANCH)to mark valid target addresses
of indirect branches (CALL, JMP etc...). If an indirect branch is
executed and the next instruction is _not_ an ENDBRANCH, the processor
generates a #CP. These instruction behaves as a NOP on platforms that
doesn't support CET.
CET states management
=====================
KVM cooperates with host kernel FPU framework to manage guest CET registers.
With CET supervisor mode state support in this series, KVM can save/restore
full guest CET xsave-managed states.
CET user mode and supervisor mode xstates, i.e., MSR_IA32_{U_CET,PL3_SSP}
and MSR_IA32_PL{0,1,2}, depend on host FPU framework to swap guest and host
xstates. On VM-Exit, guest CET xstates are saved to guest fpu area and host
CET xstates are loaded from task/thread context before vCPU returns to
userspace, vice-versa on VM-Entry. See details in kvm_{load,put}_guest_fpu().
CET supervisor mode states are grouped into two categories : XSAVE-managed
and non-XSAVE-managed, the former includes MSR_IA32_PL{0,1,2}_SSP and are
controlled by CET supervisor mode bit(S_CET bit) in XSS, the later consists
of MSR_IA32_S_CET and MSR_IA32_INTR_SSP_TBL.
VMX introduces new VMCS fields, {GUEST|HOST}_{S_CET,SSP,INTR_SSP_TABL}, to
facilitate guest/host non-XSAVES-managed states. When VMX CET entry/exit load
bits are set, guest/host MSR_IA32_{S_CET,INTR_SSP_TBL,SSP} are loaded from
equivalent fields at VM-Exit/Entry. With these new fields, such supervisor
states require no addtional KVM save/reload actions.
Tests
======
This series has successfully passed the basic CET user shadow stack test
and kernel IBT test in both L1 and L2 guests. The newly added
KVM-unit-tests [2] also passed, and its v11 has been tested with the AMD
CET series by John [3].
For your convenience, you can use my WIP QEMU [1] for testing.
[1]: https://github.com/gaochaointel/qemu-dev qemu-cet
[2]: https://lore.kernel.org/kvm/20250626073459.12990-1-minipli@xxxxxxxxxxxxxx/
[3]: https://lore.kernel.org/kvm/aH6CH+x5mCDrvtoz@xxxxxxxxxxxxxxxxxxx/
I rebased the KVM FRED patch set on top of this KVM CET patch set, and
will send out soon.
