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

[Wanpeng Li <kernellwp@gmail.com>]

On Wed, 8 Jul 2020 at 16:38, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 08/07/20 10:17, Wanpeng Li wrote:
> > On Sat, 18 Apr 2020 at 00:39, Paolo Bonzini <pbonzini@redhat.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@vger.kernel.org
> >> Signed-off-by: Paolo Bonzini <pbonzini@redhat.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?

Actually I don't know what's happening here(why not intercept hlt
instruction has associated with this commit), otherwise, it has
already been fixed. :)

    Wanpeng