Re: [PATCH v3 0/2] Fix for a very old KVM bug in the segment cache

mlevitsk
2024-08-02T15:33:29Z

У чт, 2024-07-25 у 13:52 -0400, Maxim Levitsky пише:
> Hi,
>
> Recently, while trying to understand why the pmu_counters_test
> selftest sometimes fails when run nested I stumbled
> upon a very interesting and old bug:
>
> It turns out that KVM caches guest segment state,
> but this cache doesn't have any protection against concurrent use.
>
> This usually works because the cache is per vcpu, and should
> only be accessed by vCPU thread, however there is an exception:
>
> If the full preemption is enabled in the host kernel,
> it is possible that vCPU thread will be preempted, for
> example during the vmx_vcpu_reset.
>
> vmx_vcpu_reset resets the segment cache bitmask and then initializes
> the segments in the vmcs, however if the vcpus is preempted in the
> middle of this code, the kvm_arch_vcpu_put is called which
> reads SS's AR bytes to determine if the vCPU is in the kernel mode,
> which caches the old value.
>
> Later vmx_vcpu_reset will set the SS's AR field to the correct value
> in vmcs but the cache still contains an invalid value which
> can later for example leak via KVM_GET_SREGS and such.
>
> In particular, kvm selftests will do KVM_GET_SREGS,
> and then KVM_SET_SREGS, with a broken SS's AR field passed as is,
> which will lead to vm entry failure.
>
> This issue is not a nested issue, and actually I was able
> to reproduce it on bare metal, but due to timing it happens
> much more often nested. The only requirement for this to happen
> is to have full preemption enabled in the kernel which runs the selftest.
>
> pmu_counters_test reproduces this issue well, because it creates
> lots of short lived VMs, but the issue as was noted
> about is not related to pmu.
>
> To paritally fix this issue, call vmx_segment_cache_clear
> after we done with segment register setup in vmx_vcpu_reset.
>
> V2: incorporated Paolo's suggestion of having
>     vmx_write_segment_cache_start/end functions  (thanks!)
>
> V3: reverted to a partial fix.
>
> Best regards,
>         Maxim Levitsky
>
> Maxim Levitsky (2):
>   KVM: nVMX: use vmx_segment_cache_clear
>   VMX: reset the segment cache after segment initialization in
>     vmx_vcpu_reset
>
>  arch/x86/kvm/vmx/nested.c |  3 ++-
>  arch/x86/kvm/vmx/vmx.c    | 10 +++-------
>  arch/x86/kvm/vmx/vmx.h    |  5 +++++
>  3 files changed, 10 insertions(+), 8 deletions(-)
>
> --
> 2.26.3
>
>

Any update?

Best regards,
Maxim Levitsky