[PATCH v2 0/9] KVM: x86: Fixes for (benign?) truncation bugs

Sean Christopherson
2021-04-22T02:21:34Z

Patches 01 and 02 fix theoretical bugs related to loading CRs through
the emulator. The rest of the patches are a bunch of small fixes for
cases where KVM reads/writes a 64-bit register outside of 64-bit mode.

I stumbled on this when puzzling over commit 0107973a80ad ("KVM: x86:
Introduce cr3_lm_rsvd_bits in kvm_vcpu_arch"), which stated that SEV
guests failed to boot on PCID-enabled hosts. Why only PCID hosts?

After much staring, I realized that the initial CR3 load in
rsm_enter_protected_mode() would skip the MAXPHYADDR check due to the
vCPU not being in long mode. But due to the ordering problems with
PCID, when PCID is enabled in the guest, the second load of CR3 would
be done with long mode enabled and thus hit the SEV C-bit bug.

Changing kvm_set_cr3() made me look at the callers, and seeing that
SVM didn't properly truncate the value made me look at everything else,
and here we are.

Note, I strongly suspect the emulator still has bugs. But, unless the
guest is deliberately trying to hit these types of bugs, even the ones
fixed here, they're likely benign. I figured I was more likely to break
something than I was to fix something by diving into the emulator, so I
left it alone. For now. :-)

v2: Rebase to kvm/queue, commit 89a22e37c8c2 ("KVM: avoid "deadlock"
between install_new_memslots and MMU notifier")

v1: https://lkml.kernel.org/r/20210213010518.1682691-1-seanjc@google.com

Sean Christopherson (9):
KVM: x86: Remove emulator's broken checks on CR0/CR3/CR4 loads
KVM: x86: Check CR3 GPA for validity regardless of vCPU mode
KVM: SVM: Truncate GPR value for DR and CR accesses in !64-bit mode
KVM: VMX: Truncate GPR value for DR and CR reads in !64-bit mode
KVM: nVMX: Truncate bits 63:32 of VMCS field on nested check in
!64-bit
KVM: nVMX: Truncate base/index GPR value on address calc in !64-bit
KVM: x86/xen: Drop RAX[63:32] when processing hypercall
KVM: SVM: Use default rAX size for INVLPGA emulation
KVM: x86: Rename GPR accessors to make mode-aware variants the
defaults

arch/x86/kvm/emulate.c | 68 +----------------------------------
arch/x86/kvm/kvm_cache_regs.h | 19 ++++++----
arch/x86/kvm/svm/svm.c | 12 +++++--
arch/x86/kvm/vmx/nested.c | 14 ++++----
arch/x86/kvm/vmx/vmx.c | 6 ++--
arch/x86/kvm/x86.c | 19 ++++++----
arch/x86/kvm/x86.h | 8 ++---
7 files changed, 48 insertions(+), 98 deletions(-)