[v2] KVM: x86: Question on lock protection in handle_ept_misconfig

[v2] KVM: x86: Question on lock protection in handle_ept_misconfig

[zoudongjie (A)]

Resending as plain text to fix formatting issues.
---

Hi all,

I noticed that in handle_ept_misconfig(), kvm_io_bus_write() is called. And 
within kvm_io_bus_write(), BUS is obtained through srcu_dereference(). During 
this process, kvm->slots_lock is not acquired, nor is srcu_read_lock() called 
for protection. If another process is synchronizing BUS at the same time, 
synchronize_srcu_expedited() cannot safely reclaim space(it cannot protect 
srcu_dereference() outside the critical section?), how can we ensure that BUS 
obtained by kvm_io_bus_write() is the latest?

Thanks,
Junjie Cai

Reported by: Junjie Cai <mailto:caijunjie15@h-partners.com>

Re: [v2] KVM: x86: Question on lock protection in handle_ept_misconfig

[Paolo Bonzini]

Il mer 9 lug 2025, 08:50 zoudongjie (A) <zoudongjie@huawei.com> ha scritto:
>
> Resending as plain text to fix formatting issues.
> ---
>
> Hi all,
>
> I noticed that in handle_ept_misconfig(), kvm_io_bus_write() is called. And
> within kvm_io_bus_write(), BUS is obtained through srcu_dereference(). During
> this process, kvm->slots_lock is not acquired, nor is srcu_read_lock() called
> for protection.


Hi, srcu_read_lock() is guaranteed by the caller (see vcpu_enter_guest
in arch/x86/kvm/x86.c).

In fact, almost all of KVM_RUN is covered by a SRCU read critical
section, except for:

1) the part where the processor is in guest mode and the nearby
preparations (that's vcpu_enter_guest)

2) when the vCPU goes to a non-runnable state, for example waiting for
an interrupt (search for kvm_vcpu_block and kvm_vcpu_halt)

Thanks,

Paolo

> If another process is synchronizing BUS at the same time,
> synchronize_srcu_expedited() cannot safely reclaim space(it cannot protect
> srcu_dereference() outside the critical section?), how can we ensure that BUS
> obtained by kvm_io_bus_write() is the latest?
>
> Thanks,
> Junjie Cai
>
> Reported by: Junjie Cai <mailto:caijunjie15@h-partners.com>
>