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[199.106.103.254]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-87d1cfbbfb5sm1725238b3a.18.2026.09.23.09.51.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 23 Sep 2026 09:51:31 -0700 (PDT) Date: Wed, 23 Sep 2026 09:51:26 -0700 From: Jonathan Cameron To: Suzuki K Poulose Cc: kvm@vger.kernel.org, kvmarm@lists.linux.dev, maz@kernel.org, will@kernel.org, catalin.marinas@arm.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, steven.price@arm.com, aneesh.kumar@kernel.org, oupton@kernel.org, gshan@redhat.com, joey.gouly@arm.com, tabba@google.com, yuzenghui@huawei.com, linux-coco@lists.linux.dev, gankulkarni@os.amperecomputing.com, sdonthineni@nvidia.com, alpergun@google.com, fj0570is@fujitsu.com, WeiLin.Chang@arm.com, lpieralisi@kernel.org, enju.kohei@fujitsu.com Subject: Re: [PATCH v19 01/20] KVM: arm64: protected VM: Handle user writes to CNTVCT_EL0/CNTPCT_EL0 Message-ID: <20260923095126.0000241d@oss.qualcomm.com> In-Reply-To: References: <20260920212845.707-1-suzuki.poulose@arm.com> <20260920212845.707-2-suzuki.poulose@arm.com> <20260922122542.00006868@oss.qualcomm.com> Organization: Qualcomm X-Mailer: Claws Mail 4.4.0 (GTK 3.24.51; x86_64-w64-mingw32) Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Proofpoint-Spam-Info: AW1haW4tMjYwOTIzMDA2NyBTYWx0ZWRfX/AWeNidtj7kh pS9Km94Fk+6gEpMZzUQ+9pZWxs4v1fsVBbknNqbPI+ACKjpm96hyWdqWAHKFDWXMxSui5T45MTk u/h26NGl0qN3MG8sbjuuJVDAHdKng/A= X-Authority-Analysis: v=2.4 cv=R/6WitRX c=1 sm=1 tr=0 ts=6ab40395 cx=c_pps a=WW5sKcV1LcKqjgzy2JUPuA==:117 a=JYp8KDb2vCoCEuGobkYCKw==:17 a=kj9zAlcOel0A:10 a=VdqzKS8jKosA:10 a=s4-Qcg_JpJYA:10 a=VkNPw1HP01LnGYTKEx00:22 a=u7WPNUs3qKkmUXheDGA7:22 a=yOCtJkima9RkubShWh1s:22 a=VwQbUJbxAAAA:8 a=7CQSdrXTAAAA:8 a=7RKtW5IRJYTtfXkaynwA:9 a=CjuIK1q_8ugA:10 a=OpyuDcXvxspvyRM73sMx:22 a=a-qgeE7W1pNrGK8U0ZQC:22 X-Proofpoint-GUID: dPOZWXf_xkAKVyW0Dr3P4TsjQK0IQQUo X-Proofpoint-ORIG-GUID: dPOZWXf_xkAKVyW0Dr3P4TsjQK0IQQUo X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwOTIzMDA2NyBTYWx0ZWRfX64ZOMv8GFO5b /qA/mn7A7hfZRjTYw39/WHeZy/W8EV2IQEPPZEyi/po6hvzqI9jzP2LhHodEg/vYxj7+dl/GDrV AcbxtNYrE2HIyhYG52S4D/P52VnJ3gLxS66zR2/yYyxqezGfmkGNPrE+GMRqRHygLMOmk4nlfcL sISK6msUKTWJ1+2PY1BAXeaCeFEDr9KrVuz6vKLD/5D2Ob0kwpdimW3jkcN4wWLzuMnOoh13ucg 06JzMX7PR3hxYs1jw/ZEntQfw8d/WETBuvLdDWsVotd7A2AwNiY9zPcu77dAGwhx7qCY8SKqE1m r994NmD5jaY/9pf33dLBUHyyv97PpjzpxnxyL/FMnHosuXD6mY4KYVDsEiG6QFlJ1jAWMjyOBnV csfXe0IttO7ViMrXIblKTs3V3LBcKjrBJEtNxRp0IlHxUBIGL6DWB+bCSkGgiX35rRQWtJVvN7j pe3YhblTo9cM8mJlZ6A== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-09-23_06,2026-09-21_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 clxscore=1015 malwarescore=0 impostorscore=0 priorityscore=1501 lowpriorityscore=0 spamscore=0 suspectscore=0 phishscore=0 adultscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2609040000 definitions=main-2609230067 On Tue, 22 Sep 2026 23:04:42 +0100 Suzuki K Poulose wrote: > On 22/09/2026 20:25, Jonathan Cameron wrote: > > On Sun, 20 Sep 2026 22:28:26 +0100 > > Suzuki K Poulose wrote: > > > > Hi Suzuki, > > > >> Protected VMs doesn't allow setting offsets for virtual and phyiscal > > > > physical > > > >> counters, as the offset is always fixed to 0. The VM ioclt is filtered > > > > ioctl > > > >> out based on the cap. However we don't prevent the userspace from trying > >> to write to the CNTVCT/CNTPCT registers. This would lead to KVM triggering > >> a WARN() in timer_set_offset() as the vm_offset pointer is set to NULL. > >> > >> Fix this by always "fixing" the timer offsets to 0 and marking that the > >> timer offset is set in the kvm->arch.flags at KVM init time for protected > >> VMs. A userspace writing to the CNT*CT_EL0 would observe success, without > >> any real effect. This was chosen over preventing the writes to these > >> registers and returning -EPERM. > > > > Why? I don't mind the decision but telling us what was chosen is something > > we can see in the code - patch description should give us the stuff we > > can't see. > > > > One comment on the comment below. > >> > >> Reported by Sashiko > >> > >> Link: https://lore.kernel.org/all/20260908164641.416911F00A3A@smtp.kernel.org > >> Fixes: f7d05ee84a6a ("KVM: arm64: Prevent host from managing timer offsets for protected VMs") > >> Suggested-by: Marc Zyngier > >> Signed-off-by: Suzuki K Poulose > >> --- > >> Changes since v18: > >> - Retain NULL vm_offset for protected VMs to avoid host tampering with the > >> offset. > >> - Moved the flag setting into kvm_timer_init_vm(), where it should have been > >> in the first place > >> --- > >> arch/arm64/kvm/arch_timer.c | 12 ++++++++++-- > >> 1 file changed, 10 insertions(+), 2 deletions(-) > >> > >> diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c > >> index 6ac3321f4c575..226cd5a495c8b 100644 > >> --- a/arch/arm64/kvm/arch_timer.c > >> +++ b/arch/arm64/kvm/arch_timer.c > >> @@ -1110,8 +1110,7 @@ void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu) > >> timer_context_init(vcpu, i); > >> > >> /* Synchronize offsets across timers of a VM if not already provided */ > >> - if (!vcpu_is_protected(vcpu) && > >> - !test_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET, &vcpu->kvm->arch.flags)) { > >> + if (!test_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET, &vcpu->kvm->arch.flags)) { > >> timer_set_offset(vcpu_vtimer(vcpu), kvm_phys_timer_read()); > >> timer_set_offset(vcpu_ptimer(vcpu), 0); > >> } > >> @@ -1133,6 +1132,15 @@ void kvm_timer_init_vm(struct kvm *kvm) > >> */ > >> for (int i = 0; i < NR_KVM_TIMERS; i++) > >> kvm->arch.timer_data.ppi[i] = get_vgic_ppi(kvm, default_ppi[i]); > >> + > >> + /* > >> + * Protected VMs don't allow any offset being set from userspace, > >> + * either set via writes to the counters or using the dedicated > > This sentence confused me. Second clause isn't obviously the ways that > > are being blocked. Maybe shorten to: > > > > Protected VMs don't allow the offset to be set from userspace, > > whether via writes to the counters or the dedicated ioctl. > > LLM suggests: > > > * Protected VMs don't allow userspace to set counter offsets, > * either via counter register writes or the dedicated ioctl. Hmm. I'd not combine a negative with either. Still good enough I think! Our Booker prize may be a way off yet :) > > Have updated the comment. > > Cheers > Suzuki > > > > >> + * ioctl. Pretend the offset has already been set and rely on the > >> + * default offset being 0. > >> + */ > >> + if (kvm_vm_is_protected(kvm)) > >> + set_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET, &kvm->arch.flags); > >> } > >> > >> void kvm_timer_cpu_up(void) > > >