linux-kernel - Re: [PATCH 0/9] nSVM: Security and correctness fixes

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Date:   Thu, 03 Nov 2022 16:06:40 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>,
        Yang Zhong <yang.zhong@...el.com>,
        Wei Wang <wei.w.wang@...el.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Peter Xu <peterx@...hat.com>, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 0/9] nSVM: Security and correctness fixes

On Thu, 2022-11-03 at 15:57 +0200, Maxim Levitsky wrote:
> Recently while trying to fix some unit tests I found a CVE in SVM nested code.
> 
> In 'shutdown_interception' vmexit handler we call kvm_vcpu_reset.
> 
> However if running nested and L1 doesn't intercept shutdown, we will still end
> up running this function and trigger a bug in it.
> 
> The bug is that this function resets the 'vcpu->arch.hflags' without properly
> leaving the nested state, which leaves the vCPU in inconsistent state, which
> later triggers a kernel panic in SVM code.
> 
> The same bug can likely be triggered by sending INIT via local apic to a vCPU
> which runs a nested guest.
> 
> On VMX we are lucky that the issue can't happen because VMX always intercepts
> triple faults, thus triple fault in L2 will always be redirected to L1.
> Plus the 'handle_triple_fault' of VMX doesn't reset the vCPU.
> 
> INIT IPI can't happen on VMX either because INIT events are masked while in
> VMX mode.
> 
> First 4 patches in this series address the above issue, and are
> already posted on the list with title,
> ('nSVM: fix L0 crash if L2 has shutdown condtion which L1 doesn't intercept')
> I addressed the review feedback and also added a unit test to hit this issue.
> 
> In addition to these patches I noticed that KVM doesn't honour SHUTDOWN intercept bit
> of L1 on SVM, and I included a fix to do so - its only for correctness
> as a normal hypervisor should always intercept SHUTDOWN.
> A unit test on the other hand might want to not do so.
> I also extendted the triple_fault_test selftest to hit this issue.
> 
> Finaly I found another security issue, I found a way to
> trigger a kernel non rate limited printk on SVM from the guest, and
> last patch in the series fixes that.
> 
> A unit test I posted to kvm-unit-tests project hits this issue, so
> no selftest was added.
> 
> Best regards,
>         Maxim Levitsky
> 
> Maxim Levitsky (9):
>   KVM: x86: nSVM: leave nested mode on vCPU free
>   KVM: x86: nSVM: harden svm_free_nested against freeing vmcb02 while
>     still in use
>   KVM: x86: add kvm_leave_nested
>   KVM: x86: forcibly leave nested mode on vCPU reset
>   KVM: selftests: move idt_entry to header
>   kvm: selftests: add svm nested shutdown test
>   KVM: x86: allow L1 to not intercept triple fault
>   KVM: selftests: add svm part to triple_fault_test
>   KVM: x86: remove exit_int_info warning in svm_handle_exit
> 
>  arch/x86/kvm/svm/nested.c                     | 12 +++-
>  arch/x86/kvm/svm/svm.c                        | 10 +--
>  arch/x86/kvm/vmx/nested.c                     |  4 +-
>  arch/x86/kvm/x86.c                            | 29 ++++++--
>  tools/testing/selftests/kvm/.gitignore        |  1 +
>  tools/testing/selftests/kvm/Makefile          |  1 +
>  .../selftests/kvm/include/x86_64/processor.h  | 13 ++++
>  .../selftests/kvm/lib/x86_64/processor.c      | 13 ----
>  .../kvm/x86_64/svm_nested_shutdown_test.c     | 71 +++++++++++++++++++
>  .../kvm/x86_64/triple_fault_event_test.c      | 71 ++++++++++++++-----
>  10 files changed, 174 insertions(+), 51 deletions(-)
>  create mode 100644 tools/testing/selftests/kvm/x86_64/svm_nested_shutdown_test.c
> 
> -- 
> 2.34.3
> 
> 


I jumped the gun a bit with this patch series, there are few checkpatch.pl issues and some
leftovers I didn't remove. I'll resend this shortly.

Best regards,
	Maxim Levitsky