linux-kernel - Re: [PATCH 1/2] KVM: SVM: avoid infinite loop on NPF from bad address

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Date:   Wed, 8 Jul 2020 10:38:20 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "# v3 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: SVM: avoid infinite loop on NPF from bad address

On 08/07/20 10:17, Wanpeng Li wrote:
> On Sat, 18 Apr 2020 at 00:39, Paolo Bonzini <pbonzini@...hat.com> wrote:
>> When a nested page fault is taken from an address that does not have
>> a memslot associated to it, kvm_mmu_do_page_fault returns RET_PF_EMULATE
>> (via mmu_set_spte) and kvm_mmu_page_fault then invokes svm_need_emulation_on_page_fault.
>>
>> The default answer there is to return false, but in this case this just
>> causes the page fault to be retried ad libitum.  Since this is not a
>> fast path, and the only other case where it is taken is an erratum,
>> just stick a kvm_vcpu_gfn_to_memslot check in there to detect the
>> common case where the erratum is not happening.
>>
>> This fixes an infinite loop in the new set_memory_region_test.
>>
>> Fixes: 05d5a4863525 ("KVM: SVM: Workaround errata#1096 (insn_len maybe zero on SMAP violation)")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>>  arch/x86/kvm/svm/svm.c | 7 +++++++
>>  virt/kvm/kvm_main.c    | 1 +
>>  2 files changed, 8 insertions(+)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index a91e397d6750..c86f7278509b 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -3837,6 +3837,13 @@ static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
>>         bool smap = cr4 & X86_CR4_SMAP;
>>         bool is_user = svm_get_cpl(vcpu) == 3;
>>
>> +       /*
>> +        * If RIP is invalid, go ahead with emulation which will cause an
>> +        * internal error exit.
>> +        */
>> +       if (!kvm_vcpu_gfn_to_memslot(vcpu, kvm_rip_read(vcpu) >> PAGE_SHIFT))
>> +               return true;
>> +
>>         /*
>>          * Detect and workaround Errata 1096 Fam_17h_00_0Fh.
>>          *
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index e2f60e313c87..e7436d054305 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -1602,6 +1602,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
>>  {
>>         return __gfn_to_memslot(kvm_vcpu_memslots(vcpu), gfn);
>>  }
>> +EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
> 
> This commit incurs the linux guest fails to boot once add --overcommit
> cpu-pm=on or not intercept hlt instruction, any thoughts?

Can you write a selftest?

Paolo