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

[Christian Borntraeger]

Am 14.01.22 um 14:32 schrieb Mark Rutland:
> On Fri, Jan 14, 2022 at 01:29:46PM +0100, Christian Borntraeger wrote:
> > Am 14.01.22 um 13:19 schrieb Mark Rutland:
> > > On Thu, Jan 13, 2022 at 04:20:07PM +0100, Christian Borntraeger wrote:
> > > > 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?
> > >
> > > Yes; that's much simpler to follow!
> > >
> > > One major thing I wasn't sure about for s390 is the sequence:
> > >
> > > 	guest_enter_irqoff();	// Enters an RCU EQS
> > > 	...
> > > 	local_irq_enable();
> > > 	...
> > > 	sie64a(...);
> > > 	...
> > > 	local_irq_disable();
> > > 	...
> > > 	guest_exit_irqoff();	// Exits an RCU EQS
> > >
> > > ... since if an IRQ is taken between local_irq_{enable,disable}(), RCU won't be
> > > watching, and I couldn't spot whether your regular IRQ entry logic would wake
> > > RCU in this case, or whether there was something else I'm missing that saves
> > > you here.
> > >
> > > For other architectures, including x86 and arm64, we enter the guest with IRQs
> > > masked and return from the guest with IRQs masked, and don't actually take IRQs
> > > until we unmask them in the host, after the guest_exit_*() logic has woken RCU
> > > and so on.
> > >
> > > I wasn't able to find documentation on the semantics of SIE, so I couldn't spot
> > > whether the local_irq_{enable,disable}() calls were necessary, or could be
> > > removed.
> >
> > We run the SIE instruction with interrupts enabled. SIE is interruptible.
> > The disable/enable pairs are just because  guest_enter/exit_irqoff() require them.
>
> What I was trying to figure out was when an interrupt is taken between
> guest_enter_irqoff() and guest_exit_irqoff(), where is RCU woken? I couldn't
> spot that in the s390 entry code (probably simply because I'm not familiar with
> it), and so AFAICT that means IRQ code could run without RCU watching, which
> would cause things to explode.
>
> On other architectures that problem is avoided because IRQs asserted during the
> guest cause a specific guest exit rather than a regular IRQ exception, and the
> HW enables/disables IRQs when entering/exiting the guest, so the host can leave
> IRQs masked across guest_enter_irqoff()..guest_exit_irqoff().
>
> Am I right in understanding that SIE itself won't enable (host) interrupts
> while running the guest, and so it *needs* to be run with interrupts already
> enabled?

yes

> > One thing to be aware of: in our entry.S - after an interrupt - we leave SIE by
> > setting the return address of the interrupt after the sie instruction so that we
> > get back into this __vcpu_run loop to check for signals and so.
>
> Just to check, that's after the IRQ handler runs, right?

and yes.