From: Paolo Bonzini pbonzini@redhat.com
commit e61ab42de874c5af8c5d98b327c77a374d9e7da1 upstream.
It is error-prone that code after vmexit cannot access percpu data because GSBASE has not been restored yet. It forces MSR_IA32_SPEC_CTRL save/restore to happen very late, after the predictor untraining sequence, and it gets in the way of return stack depth tracking (a retbleed mitigation that is in linux-next as of 2022-11-09).
As a first step towards fixing that, move the VMCB VMSAVE/VMLOAD to assembly, essentially undoing commit fb0c4a4fee5a ("KVM: SVM: move VMLOAD/VMSAVE to C code", 2021-03-15). The reason for that commit was that it made it simpler to use a different VMCB for VMLOAD/VMSAVE versus VMRUN; but that is not a big hassle anymore thanks to the kvm-asm-offsets machinery and other related cleanups.
The idea on how to number the exception tables is stolen from a prototype patch by Peter Zijlstra.
Cc: stable@vger.kernel.org Fixes: a149180fbcf3 ("x86: Add magic AMD return-thunk") Link: https://lore.kernel.org/all/f571e404-e625-bae1-10e9-449b2eb4cbd8@citrix.com/ Reviewed-by: Sean Christopherson seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/kvm-asm-offsets.c | 1 arch/x86/kvm/svm/svm.c | 9 ------- arch/x86/kvm/svm/vmenter.S | 49 +++++++++++++++++++++++++++++++---------- 3 files changed, 39 insertions(+), 20 deletions(-)
--- a/arch/x86/kvm/kvm-asm-offsets.c +++ b/arch/x86/kvm/kvm-asm-offsets.c @@ -16,6 +16,7 @@ static void __used common(void) BLANK(); OFFSET(SVM_vcpu_arch_regs, vcpu_svm, vcpu.arch.regs); OFFSET(SVM_current_vmcb, vcpu_svm, current_vmcb); + OFFSET(SVM_vmcb01, vcpu_svm, vmcb01); OFFSET(KVM_VMCB_pa, kvm_vmcb_info, pa); }
--- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3923,16 +3923,7 @@ static noinstr void svm_vcpu_enter_exit( } else { struct svm_cpu_data *sd = per_cpu(svm_data, vcpu->cpu);
- /* - * Use a single vmcb (vmcb01 because it's always valid) for - * context switching guest state via VMLOAD/VMSAVE, that way - * the state doesn't need to be copied between vmcb01 and - * vmcb02 when switching vmcbs for nested virtualization. - */ - vmload(svm->vmcb01.pa); __svm_vcpu_run(svm); - vmsave(svm->vmcb01.pa); - vmload(__sme_page_pa(sd->save_area)); }
--- a/arch/x86/kvm/svm/vmenter.S +++ b/arch/x86/kvm/svm/vmenter.S @@ -28,6 +28,8 @@ #define VCPU_R15 (SVM_vcpu_arch_regs + __VCPU_REGS_R15 * WORD_SIZE) #endif
+#define SVM_vmcb01_pa (SVM_vmcb01 + KVM_VMCB_pa) + .section .noinstr.text, "ax"
/** @@ -55,6 +57,16 @@ SYM_FUNC_START(__svm_vcpu_run) mov %_ASM_ARG1, %_ASM_DI .endif
+ /* + * Use a single vmcb (vmcb01 because it's always valid) for + * context switching guest state via VMLOAD/VMSAVE, that way + * the state doesn't need to be copied between vmcb01 and + * vmcb02 when switching vmcbs for nested virtualization. + */ + mov SVM_vmcb01_pa(%_ASM_DI), %_ASM_AX +1: vmload %_ASM_AX +2: + /* Get svm->current_vmcb->pa into RAX. */ mov SVM_current_vmcb(%_ASM_DI), %_ASM_AX mov KVM_VMCB_pa(%_ASM_AX), %_ASM_AX @@ -80,16 +92,11 @@ SYM_FUNC_START(__svm_vcpu_run) /* Enter guest mode */ sti
-1: vmrun %_ASM_AX - -2: cli - -#ifdef CONFIG_RETPOLINE - /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */ - FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE -#endif +3: vmrun %_ASM_AX +4: + cli
- /* "POP" @svm to RAX. */ + /* Pop @svm to RAX while it's the only available register. */ pop %_ASM_AX
/* Save all guest registers. */ @@ -110,6 +117,18 @@ SYM_FUNC_START(__svm_vcpu_run) mov %r15, VCPU_R15(%_ASM_AX) #endif
+ /* @svm can stay in RDI from now on. */ + mov %_ASM_AX, %_ASM_DI + + mov SVM_vmcb01_pa(%_ASM_DI), %_ASM_AX +5: vmsave %_ASM_AX +6: + +#ifdef CONFIG_RETPOLINE + /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */ + FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE +#endif + /* * Mitigate RETBleed for AMD/Hygon Zen uarch. RET should be * untrained as soon as we exit the VM and are back to the @@ -159,11 +178,19 @@ SYM_FUNC_START(__svm_vcpu_run) pop %_ASM_BP RET
-3: cmpb $0, kvm_rebooting +10: cmpb $0, kvm_rebooting jne 2b ud2 +30: cmpb $0, kvm_rebooting + jne 4b + ud2 +50: cmpb $0, kvm_rebooting + jne 6b + ud2
- _ASM_EXTABLE(1b, 3b) + _ASM_EXTABLE(1b, 10b) + _ASM_EXTABLE(3b, 30b) + _ASM_EXTABLE(5b, 50b)
SYM_FUNC_END(__svm_vcpu_run)