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AFNElJ/b7X918lVv6u8+SOykwNCF0fFOWeUS2UyhDAeYlYTcztswtINhxj4EjRVie6Mt3TSp2Cw=@vger.kernel.org X-Gm-Message-State: AOJu0YwZHas2UTkcSwqeNTdKPUJAYN02gX7h9SQ6Tcs0tEXlOwQUvGCa /eyx6BqG7MQekU9cEmfv/L8Ycuakqn5MVMVzABjAm+g7IiERCLyI3IVxUHvG8SbE/xjxLgxkL/1 aJ+GE4w== X-Received: from pjot6.prod.google.com ([2002:a17:90a:9506:b0:36b:a8c4:4c94]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:2b86:b0:36a:8519:a4e9 with SMTP id 98e67ed59e1d1-37133569a9bmr12370219a91.18.1780935454108; Mon, 08 Jun 2026 09:17:34 -0700 (PDT) Date: Mon, 8 Jun 2026 09:17:33 -0700 In-Reply-To: <87o6hlhuz5.fsf@redhat.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <87o6hlhuz5.fsf@redhat.com> Message-ID: Subject: Re: [PATCH] KVM: x86: hyper-v: Bound the bank index in hv_is_vp_in_sparse_set() From: Sean Christopherson To: Vitaly Kuznetsov Cc: Hyunwoo Kim , pbonzini@redhat.com, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, kvm@vger.kernel.org, stable@vger.kernel.org Content-Type: text/plain; charset="us-ascii" On Mon, Jun 08, 2026, Vitaly Kuznetsov wrote: > Hyunwoo Kim writes: > > > hv_is_vp_in_sparse_set() uses valid_bit_nr, i.e. vp_id divided by > > HV_VCPUS_PER_SPARSE_BANK, as the test_bit() index into > > valid_bank_mask. valid_bank_mask is a single u64 and a sparse vCPU > > set holds at most HV_MAX_SPARSE_VCPU_BANKS banks, so valid_bit_nr > > must be less than HV_MAX_SPARSE_VCPU_BANKS. > > > > The caller in kvm_hv_send_ipi_to_many() passes kvm_hv_get_vpindex(), > > which is below KVM_MAX_VCPUS and therefore always within that bound. > > The L2 direct flush branch in kvm_hv_flush_tlb(), however, passes > > hv_v->nested.vp_id, copied verbatim from the enlightened VMCS > > without any bounds check, so valid_bit_nr can reach > > HV_MAX_SPARSE_VCPU_BANKS or more and test_bit() then reads beyond > > valid_bank_mask. > > > > Return false before the test_bit() when valid_bit_nr is not below > > HV_MAX_SPARSE_VCPU_BANKS, since such a VP cannot be present in the > > set. > > > > Cc: stable@vger.kernel.org > > Fixes: c58a318f6090 ("KVM: x86: hyper-v: L2 TLB flush") > > Signed-off-by: Hyunwoo Kim > > --- > > arch/x86/kvm/hyperv.c | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > > index 4438ecac9a89..d8782cb7ba02 100644 > > --- a/arch/x86/kvm/hyperv.c > > +++ b/arch/x86/kvm/hyperv.c > > @@ -1839,6 +1839,10 @@ static bool hv_is_vp_in_sparse_set(u32 vp_id, u64 valid_bank_mask, u64 sparse_ba > > int valid_bit_nr = vp_id / HV_VCPUS_PER_SPARSE_BANK; > > unsigned long sbank; > > > > + /* A bank index beyond the mask can't be set, the VP isn't in the set. */ > > + if (valid_bit_nr >= HV_MAX_SPARSE_VCPU_BANKS) > > + return false; > > + > > if (!test_bit(valid_bit_nr, (unsigned long *)&valid_bank_mask)) > > return false; > > I think the concern is valid, so Yeah, easy to trigger with KASAN and: diff --git tools/testing/selftests/kvm/x86/hyperv_evmcs.c tools/testing/selftests/kvm/x86/hyperv_evmcs.c index c7fa114aee20..0cf5f891a20d 100644 --- tools/testing/selftests/kvm/x86/hyperv_evmcs.c +++ tools/testing/selftests/kvm/x86/hyperv_evmcs.c @@ -59,6 +59,10 @@ void l2_guest_code(void) vmcall(); rdmsr_from_l2(MSR_GS_BASE); /* intercepted */ + asm volatile ("movq %0, %%xmm0" :: "r"(-1ull)); + hyperv_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE | HV_HYPERCALL_FAST_BIT, 0x0, + HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES); + /* L2 TLB flush tests */ hyperv_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE | HV_HYPERCALL_FAST_BIT, 0x0, HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES | HV_FLUSH_ALL_PROCESSORS); @@ -117,7 +121,7 @@ void guest_code(struct vmx_pages *vmx_pages, struct hyperv_test_pages *hv_pages, current_evmcs->partition_assist_page = hv_pages->partition_assist_gpa; current_evmcs->hv_enlightenments_control.nested_flush_hypercall = 1; current_evmcs->hv_vm_id = 1; - current_evmcs->hv_vp_id = 1; + current_evmcs->hv_vp_id = -1; current_vp_assist->nested_control.features.directhypercall = 1; *(u32 *)(hv_pages->partition_assist) = 0; > Reviewed-by: Vitaly Kuznetsov > > what I'm not sure about if we should also deliberately crash the VM > which does such a hypercall. This way it would be easier to find buggy > L1s but given that they are most likely Windows, we need to do some > tests to see if this is not actually happening today (e.g. Hyper-V usign > VP_ID or '-1' for something). Let's have this as a future TODO item. +1