6.12-stable review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson seanjc@google.com
commit be45bc4eff33d9a7dae84a2150f242a91a617402 upstream.
Enable/disable local IRQs, i.e. set/clear RFLAGS.IF, in the common svm_vcpu_enter_exit() just after/before guest_state_{enter,exit}_irqoff() so that VMRUN is not executed in an STI shadow. AMD CPUs have a quirk (some would say "bug"), where the STI shadow bleeds into the guest's intr_state field if a #VMEXIT occurs during injection of an event, i.e. if the VMRUN doesn't complete before the subsequent #VMEXIT.
The spurious "interrupts masked" state is relatively benign, as it only occurs during event injection and is transient. Because KVM is already injecting an event, the guest can't be in HLT, and if KVM is querying IRQ blocking for injection, then KVM would need to force an immediate exit anyways since injecting multiple events is impossible.
However, because KVM copies int_state verbatim from vmcb02 to vmcb12, the spurious STI shadow is visible to L1 when running a nested VM, which can trip sanity checks, e.g. in VMware's VMM.
Hoist the STI+CLI all the way to C code, as the aforementioned calls to guest_state_{enter,exit}_irqoff() already inform lockdep that IRQs are enabled/disabled, and taking a fault on VMRUN with RFLAGS.IF=1 is already possible. I.e. if there's kernel code that is confused by running with RFLAGS.IF=1, then it's already a problem. In practice, since GIF=0 also blocks NMIs, the only change in exposure to non-KVM code (relative to surrounding VMRUN with STI+CLI) is exception handling code, and except for the kvm_rebooting=1 case, all exception in the core VM-Enter/VM-Exit path are fatal.
Use the "raw" variants to enable/disable IRQs to avoid tracing in the "no instrumentation" code; the guest state helpers also take care of tracing IRQ state.
Oppurtunstically document why KVM needs to do STI in the first place.
Reported-by: Doug Covelli doug.covelli@broadcom.com Closes: https://lore.kernel.org/all/CADH9ctBs1YPmE4aCfGPNBwA10cA8RuAk2gO7542DjMZgs4u... Fixes: f14eec0a3203 ("KVM: SVM: move more vmentry code to assembly") Cc: stable@vger.kernel.org Reviewed-by: Jim Mattson jmattson@google.com Link: https://lore.kernel.org/r/20250224165442.2338294-2-seanjc@google.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/svm/svm.c | 14 ++++++++++++++ arch/x86/kvm/svm/vmenter.S | 10 +--------- 2 files changed, 15 insertions(+), 9 deletions(-)
--- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -4176,6 +4176,18 @@ static noinstr void svm_vcpu_enter_exit(
guest_state_enter_irqoff();
+ /* + * Set RFLAGS.IF prior to VMRUN, as the host's RFLAGS.IF at the time of + * VMRUN controls whether or not physical IRQs are masked (KVM always + * runs with V_INTR_MASKING_MASK). Toggle RFLAGS.IF here to avoid the + * temptation to do STI+VMRUN+CLI, as AMD CPUs bleed the STI shadow + * into guest state if delivery of an event during VMRUN triggers a + * #VMEXIT, and the guest_state transitions already tell lockdep that + * IRQs are being enabled/disabled. Note! GIF=0 for the entirety of + * this path, so IRQs aren't actually unmasked while running host code. + */ + raw_local_irq_enable(); + amd_clear_divider();
if (sev_es_guest(vcpu->kvm)) @@ -4184,6 +4196,8 @@ static noinstr void svm_vcpu_enter_exit( else __svm_vcpu_run(svm, spec_ctrl_intercepted);
+ raw_local_irq_disable(); + guest_state_exit_irqoff(); }
--- a/arch/x86/kvm/svm/vmenter.S +++ b/arch/x86/kvm/svm/vmenter.S @@ -170,12 +170,8 @@ SYM_FUNC_START(__svm_vcpu_run) mov VCPU_RDI(%_ASM_DI), %_ASM_DI
/* Enter guest mode */ - sti - 3: vmrun %_ASM_AX 4: - cli - /* Pop @svm to RAX while it's the only available register. */ pop %_ASM_AX
@@ -340,12 +336,8 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run) mov KVM_VMCB_pa(%rax), %rax
/* Enter guest mode */ - sti - 1: vmrun %rax - -2: cli - +2: /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */ FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT