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Tue, 28 Jul 2026 15:09:53 +0000 (GMT) Received: from [9.111.53.50] (unknown [9.111.53.50]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Tue, 28 Jul 2026 15:09:53 +0000 (GMT) Message-ID: Date: Tue, 28 Jul 2026 17:09:53 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 01/10] KVM: s390: Fix unlikely NULL gmap dereference To: Claudio Imbrenda , linux-kernel@vger.kernel.org Cc: kvm@vger.kernel.org, linux-s390@vger.kernel.org, frankja@linux.ibm.com, david@kernel.org, seiden@linux.ibm.com, nrb@linux.ibm.com, schlameuss@linux.ibm.com, gra@linux.ibm.com References: <20260727150254.236021-1-imbrenda@linux.ibm.com> <20260727150254.236021-2-imbrenda@linux.ibm.com> Content-Language: en-US From: Christian Borntraeger In-Reply-To: <20260727150254.236021-2-imbrenda@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: oRf52uvLjiXImTUJylwVEYMw9o1uCLkr X-Proofpoint-Spam-Info: AW1haW4tMjYwNzI4MDEzMiBTYWx0ZWRfXyAQKPLjtjKvJ 1VJOtp/4EUSPe6LMIOBJP4vTstab0fT4M3B2SjUZU6AINZM5L3C7XLNAeKfpIUxtMTKW/HOzW4v fi7jUXeKc5hkx6K1jsSn5GJit8vfkuI= X-Authority-Analysis: v=2.4 cv=E/z9Y6dl c=1 sm=1 tr=0 ts=6a68c647 cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=IkcTkHD0fZMA:10 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=VnNF1IyMAAAA:8 a=_YX56pO25_faRPOBtT8A:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzI4MDEzMiBTYWx0ZWRfX//SAZo6s52Nm G1TH+66me4FWINNboBdVgworbhpl6Rexc7gcVS/bIdBGpx4FOMkaCW352279EFypQkZOLfFnwdo xjROHOslThym5UPCsFbysjAIjAsQSSjJBMWOfCBUkXgmvP21DbeW0PpDzWQLzFMlFIKxMsoFrj6 PVf6EZa35KJrhwFrM+M7XminQHGDQQpFNH2WdUcNmk+dMbI7PNEK5FEO+kW9+AuUtOe3LSGAGjv zDITDoRrNIcAbnEyz5Wu2+DmHROBUx7/cIV/Bmr/oFuFXm1fXq5kG0lI0iSvFr+yUoSeD5ylGtH Te7hWdkPx94IGOKHSAlLedg7YIiDnXRXC3wT2CGRwvr1bXvmfX7f3+Vd1WWCOdgGL62tig1nuxP lgsJzP9qCZqmkOddrqfnFoqxqGgD5HB2raweybBJ7j6dPpUCVfQoM72FUqCR7afwv02TfKea1i6 hgzOqOZXLIm3yHuxOwA== X-Proofpoint-GUID: oRf52uvLjiXImTUJylwVEYMw9o1uCLkr X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-07-28_04,2026-07-28_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 priorityscore=1501 spamscore=0 clxscore=1011 phishscore=0 lowpriorityscore=0 bulkscore=0 malwarescore=0 impostorscore=0 suspectscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607280132 Am 27.07.26 um 17:02 schrieb Claudio Imbrenda: > When creating a new vCPU, kvm_vm_ioctl_create_vcpu() will call > kvm_arch_vcpu_postcreate() after the file descriptor for the new vCPU > has been created. The new file descriptor has not been returned yet, > but a malicious userspace program could try to guess it. > > If a malicious userspace program manages to start the newly created vCPU > before kvm_arch_vcpu_postcreate() is called, __vcpu_run() will try to > dereference vcpu->arch.gmap and trigger a NULL pointer dereference. > > Fix this by adding a new field to struct kvm_vcpu_arch to keep track of > the initialization status of the vCPU. Refuse to run a vCPU that is not > fully initialized. > > Fixes: dafd032a15f8 ("KVM: s390: move vcpu specific initalization to a later point") > Fixes: e38c884df921 ("KVM: s390: Switch to new gmap") > Signed-off-by: Claudio Imbrenda > Reviewed-by: Steffen Eiden > --- > arch/s390/include/asm/kvm_host.h | 1 + > arch/s390/kvm/kvm-s390.c | 8 ++++++++ > 2 files changed, 9 insertions(+) > > diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h > index eaa34c5bd3c1..edf75b6ad20c 100644 > --- a/arch/s390/include/asm/kvm_host.h > +++ b/arch/s390/include/asm/kvm_host.h > @@ -440,6 +440,7 @@ struct kvm_vcpu_arch { > bool skey_enabled; > /* Indicator if the access registers have been loaded from guest */ > bool acrs_loaded; > + bool initialized; > struct kvm_s390_pv_vcpu pv; > union diag318_info diag318_info; > struct kvm_s390_mmu_cache *mc; > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c > index 150b5dd2170e..22535f76ae05 100644 > --- a/arch/s390/kvm/kvm-s390.c > +++ b/arch/s390/kvm/kvm-s390.c > @@ -3613,6 +3613,8 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu) > if (test_kvm_facility(vcpu->kvm, 74) || vcpu->kvm->arch.user_instr0 || > vcpu->kvm->arch.user_operexec) > vcpu->arch.sie_block->ictl |= ICTL_OPEREXC; > + > + WRITE_ONCE(vcpu->arch.initialized, true); while highly unlikely, I think sashiko is right, the compiler could reoder the gmap assignment. A barrier() before the WRITE_ONCE would do but smp_store_release does not hurt (and it has some sanitizer support). > }> > static bool kvm_has_pckmo_subfunc(struct kvm *kvm, unsigned long nr) > @@ -5039,6 +5041,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu) > kvm_run->kvm_dirty_regs & ~KVM_SYNC_S390_VALID_FIELDS) > return -EINVAL; > > + if (!READ_ONCE(vcpu->arch.initialized)) > + return -EINVAL; > + I think we really do not need smp_load_acquire here and below, but it certainly is nicer to have equivalent pairs. So if you use a barrier, its fine to leave it as is, but maybe simply use smp_load_acquire here and below and smp_store_release above. > vcpu_load(vcpu); > > if (guestdbg_exit_pending(vcpu)) { > @@ -5523,6 +5528,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp, > long r; > u16 rc, rrc; > > + if (!READ_ONCE(vcpu->arch.initialized)) > + return -EINVAL; > + > vcpu_load(vcpu); > > switch (ioctl) {