linux-kernel - Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs

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Date:   Thu, 13 Jan 2022 16:20:07 +0100
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org
Cc:     aleksandar.qemu.devel@...il.com, alexandru.elisei@....com,
        anup.patel@....com, aou@...s.berkeley.edu, atish.patra@....com,
        benh@...nel.crashing.org, bp@...en8.de, catalin.marinas@....com,
        chenhuacai@...nel.org, dave.hansen@...ux.intel.com,
        david@...hat.com, frankja@...ux.ibm.com, frederic@...nel.org,
        gor@...ux.ibm.com, hca@...ux.ibm.com, imbrenda@...ux.ibm.com,
        james.morse@....com, jmattson@...gle.com, joro@...tes.org,
        kvm@...r.kernel.org, maz@...nel.org, mingo@...hat.com,
        mpe@...erman.id.au, nsaenzju@...hat.com, palmer@...belt.com,
        paulmck@...nel.org, paulus@...ba.org, paul.walmsley@...ive.com,
        pbonzini@...hat.com, seanjc@...gle.com, suzuki.poulose@....com,
        tglx@...utronix.de, tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs



Am 11.01.22 um 16:35 schrieb Mark Rutland:
> Several architectures have latent bugs around guest entry/exit, most
> notably:
> 
> 1) Several architectures enable interrupts between guest_enter() and
>     guest_exit(). As this period is an RCU extended quiescent state (EQS) this
>     is unsound unless the irq entry code explicitly wakes RCU, which most
>     architectures only do for entry from usersapce or idle.
> 
>     I believe this affects: arm64, riscv, s390
> 
>     I am not sure about powerpc.
> 
> 2) Several architectures permit instrumentation of code between
>     guest_enter() and guest_exit(), e.g. KASAN, KCOV, KCSAN, etc. As
>     instrumentation may directly o indirectly use RCU, this has the same
>     problems as with interrupts.
> 
>     I believe this affects: arm64, mips, powerpc, riscv, s390
> 
> 3) Several architectures do not inform lockdep and tracing that
>     interrupts are enabled during the execution of the guest, or do so in
>     an incorrect order. Generally
>     this means that logs will report IRQs being masked for much longer
>     than is actually the case, which is not ideal for debugging. I don't
>     know whether this affects the correctness of lockdep.
> 
>     I believe this affects: arm64, mips, powerpc, riscv, s390
> 
> This was previously fixed for x86 specifically in a series of commits:
> 
>    87fa7f3e98a1310e ("x86/kvm: Move context tracking where it belongs")
>    0642391e2139a2c1 ("x86/kvm/vmx: Add hardirq tracing to guest enter/exit")
>    9fc975e9efd03e57 ("x86/kvm/svm: Add hardirq tracing on guest enter/exit")
>    3ebccdf373c21d86 ("x86/kvm/vmx: Move guest enter/exit into .noinstr.text")
>    135961e0a7d555fc ("x86/kvm/svm: Move guest enter/exit into .noinstr.text")
>    160457140187c5fb ("KVM: x86: Defer vtime accounting 'til after IRQ handling")
>    bc908e091b326467 ("KVM: x86: Consolidate guest enter/exit logic to common helpers")
> 
> But other architectures were left broken, and the infrastructure for
> handling this correctly is x86-specific.
> 
> This series introduces generic helper functions which can be used to
> handle the problems above, and migrates architectures over to these,
> fixing the latent issues.
> 
> I wasn't able to figure my way around powerpc and s390, so I have not

I think 2 later patches have moved the guest_enter/exit a bit out.
Does this make the s390 code clearer?

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 577f1ead6a51..5859207c2cc0 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4145,10 +4145,6 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
                  * As PF_VCPU will be used in fault handler, between
                  * guest_enter and guest_exit should be no uaccess.
                  */
-               local_irq_disable();
-               guest_enter_irqoff();
-               __disable_cpu_timer_accounting(vcpu);
-               local_irq_enable();
                 if (kvm_s390_pv_cpu_is_protected(vcpu)) {
                         memcpy(sie_page->pv_grregs,
                                vcpu->run->s.regs.gprs,
@@ -4156,8 +4152,16 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
                 }
                 if (test_cpu_flag(CIF_FPU))
                         load_fpu_regs();
+               local_irq_disable();
+               __disable_cpu_timer_accounting(vcpu);
+               guest_enter_irqoff();
+               local_irq_enable();
                 exit_reason = sie64a(vcpu->arch.sie_block,
                                      vcpu->run->s.regs.gprs);
+               local_irq_disable();
+               guest_exit_irqoff();
+               __enable_cpu_timer_accounting(vcpu);
+               local_irq_enable();
                 if (kvm_s390_pv_cpu_is_protected(vcpu)) {
                         memcpy(vcpu->run->s.regs.gprs,
                                sie_page->pv_grregs,
@@ -4173,10 +4177,6 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
                                 vcpu->arch.sie_block->gpsw.mask &= ~PSW_INT_MASK;
                         }
                 }
-               local_irq_disable();
-               __enable_cpu_timer_accounting(vcpu);
-               guest_exit_irqoff();
-               local_irq_enable();
                 vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
  
                 rc = vcpu_post_run(vcpu, exit_reason);