From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx0b-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com [148.163.158.5]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 871593DE456; Wed, 5 Aug 2026 08:41:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=148.163.158.5 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785919291; cv=none; b=S2MbjYaZsIUdjjf9MLQoTb2rvvXeFAipkFwUlYYNtGmdwKhPcsC9De7TX41BvjXgyZcCNOBPmF8OaCajGnFzQTs8qwXip8i39l5ZyQgZQyRO34ukb1j+K4sJttr7iAmcsFwh3poHcrpDDyd3N3OdaPkqjRII2s0wzoK26ootAKY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785919291; c=relaxed/simple; bh=N/8smBpL3nhgl1cRNg6zSWlN20EhhEUppjKtY7oe+aI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=vErmUz/ehDrkDkvD3o/puSwc64AdBOIP0tGL0mD7wwR+lAdQEIDJ0jMFfSCFm2MIecKL4hYYJfnwb/qxOoOkSpTFOoBvDPWPZaj+OlgjmFbfjo9rDmYrldAQTAheZrKNCIo8p24o6vp+NujpDyd3bgwOdoStM7lxbCM6oVXqxwY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.ibm.com; spf=pass smtp.mailfrom=linux.ibm.com; dkim=pass (2048-bit key) header.d=ibm.com header.i=@ibm.com header.b=NbcQw2yX; arc=none smtp.client-ip=148.163.158.5 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.ibm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.ibm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=ibm.com header.i=@ibm.com header.b="NbcQw2yX" Received: from pps.filterd (m0353725.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.18.1.11/8.18.1.11) with ESMTP id 6755mCq72909930; Wed, 5 Aug 2026 08:41:27 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=cc :content-transfer-encoding:content-type:date:from:in-reply-to :message-id:mime-version:references:subject:to; s=pp1; bh=L+qstq //dyXpozpdIYKJD4rprOP3/zuOpcK8yHLhFVY=; b=NbcQw2yX6Elv710yzUrRjf WqpT42N8G7Pt69mkFqxRdlK0FGzZ2co5HFCvnryOcV2PlY707UADEBYgs6/cZgRV lOv8Q9Lfb6gr3Ld+oTJUUL4ZY3RSIIhWB/LxbjaYPWzCVOJMzRj0S/NoICn3Y+Zb /XVUueYk8pZiYDjXntzEGzHA+77W8NzKTi+NPsq/OmWm/evYJkLyOfv79GkNlb4v UOAqhEc/amtpKIgD8K85JHRq9oGN742vcyzxtipP01yJq8zt+bbL/zurbtGXM/58 yG4gdmG3T4o5SV2AEYa7Im4U4WArxIUsGx6uYaqlwq1B6ZT/9GQCctYuayeeFptA == Received: from ppma22.wdc07v.mail.ibm.com (5c.69.3da9.ip4.static.sl-reverse.com [169.61.105.92]) by mx0a-001b2d01.pphosted.com (PPS) with ESMTPS id 4fs77g9stb-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 05 Aug 2026 08:41:26 +0000 (GMT) Received: from pps.filterd (ppma22.wdc07v.mail.ibm.com [127.0.0.1]) by ppma22.wdc07v.mail.ibm.com (8.18.1.7/8.18.1.7) with ESMTP id 6758QJAX023752; Wed, 5 Aug 2026 08:41:26 GMT Received: from smtprelay04.fra02v.mail.ibm.com ([9.218.2.228]) by ppma22.wdc07v.mail.ibm.com (PPS) with ESMTPS id 4fsugw5vax-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 05 Aug 2026 08:41:25 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (smtpav02.fra02v.mail.ibm.com [10.20.54.101]) by smtprelay04.fra02v.mail.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 6758fLaL31588944 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 5 Aug 2026 08:41:22 GMT Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id CD1EB2004B; Wed, 5 Aug 2026 08:41:21 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8FBB820040; Wed, 5 Aug 2026 08:41:21 +0000 (GMT) Received: from [9.224.77.173] (unknown [9.224.77.173]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 5 Aug 2026 08:41:21 +0000 (GMT) Message-ID: Date: Wed, 5 Aug 2026 10:21:41 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 6/7] KVM: s390: Take srcu when importing watchpoint data To: sashiko-reviews@lists.linux.dev, Paolo Bonzini Cc: Vasily Gorbik , linux-s390@vger.kernel.org, kvm@vger.kernel.org, Alexander Gordeev , Heiko Carstens References: <20260804162435.28617-1-borntraeger@linux.ibm.com> <20260804162435.28617-7-borntraeger@linux.ibm.com> <20260804184024.B9A961F000E9@smtp.kernel.org> Content-Language: en-US From: Christian Borntraeger In-Reply-To: <20260804184024.B9A961F000E9@smtp.kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODA1MDA2NCBTYWx0ZWRfX0canNU0jnPmw 7+UPzm4Hu6XddtIMEyCMmKqssapoBaMPs1BDEtgdCZpQnyhofM8jn0Ou3Xj7rqjXvBdaijHLzkQ bRg1iJWjECjzPwHB/5+XNOsElw8kI0y1UEmzOVzxg1/ZSOVXgfRlHNtEplbV6BVPOr8vfFG8Kei Rs+S0yrFTZhViFrxvbe2dMso5NP67GVJkDwVuTjPTgJs0K7bn54i7W76jzrWfztN8fRlc3u97+b lkHP66QBmS9O69pJTx9Fk/A7Rg0U4I9Hw39muDz3bkO7wmeiPba10YQrW67x+ob2k6WQRmXm8SO lbeNyZ+rMrAqMFggUi7UNEVDIcnMTR5xL9ebFd+KslY6HOwRt4Mq7xFIW0qB1GFTnAxhOhSCl2n /PKK2uQGPo5PCme+izmwsJD8L83Y043hE4hb/tpFYCJIbbqHSY4ZBLGXh4yZ7+JCYw+Wg60xEJz 0usdzdeWw2FqqK8TGXw== X-Authority-Analysis: v=2.4 cv=WIFPmHsR c=1 sm=1 tr=0 ts=6a72f736 cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VwQbUJbxAAAA:8 a=CqDQVMNXfGTbBfhByaQA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: I3wRAU3sU_BApMvTViLDZlucNOwDRvAR X-Proofpoint-ORIG-GUID: I3wRAU3sU_BApMvTViLDZlucNOwDRvAR X-Proofpoint-Spam-Info: AW1haW4tMjYwODA1MDA2NCBTYWx0ZWRfXx5Y/wssZU1AZ Eot5B2AALe30BXfIVeO+01BGVEwr9OWk+8H6QrC2rjELLi/sg+CarkRBLdloKZfX1RtBA3ce5Kv bZ7TA+uVL0FSQZ46QEKFR4cRlKQ9cLs= 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=2026-08-05_02,2026-08-04_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 lowpriorityscore=0 priorityscore=1501 phishscore=0 malwarescore=0 suspectscore=0 clxscore=1015 impostorscore=0 bulkscore=0 adultscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608050064 Am 04.08.26 um 20:40 schrieb sashiko-bot@kernel.org: > > [Severity: High] > Could holding the SRCU read lock across this call introduce a userfaultfd > deadlock? > > Inside kvm_s390_import_bp_data(), the very first operation is: > > arch/s390/kvm/guestdbg.c:kvm_s390_import_bp_data() { > ... > bp_data = memdup_array_user(dbg->arch.hw_bp, dbg->arch.nr_hw_bp, > sizeof(*bp_data)); > ... > } > > If user space maps this array using userfaultfd, the VCPU thread will block > in copy_from_user() while holding the kvm->srcu read lock. If the user space > userfaultfd handler attempts to execute a VM-wide KVM ioctl that modifies > memslots (such as KVM_SET_USER_MEMORY_REGION), that ioctl will call > synchronize_srcu(&kvm->srcu). The VCPU thread would wait for the page fault > to be resolved, and the userfaultfd handler would wait for the SRCU lock to > be released. Now I am puzzled. Sashiko basically asked for srcu protection for the guest access in the same review series. But I want to followup nevertheless. There are other places in KVM code which do access guest memory under the srcu. 1. KVM_SET_NESTED_STATE — arch/x86/kvm/x86.c idx = srcu_read_lock(&vcpu->kvm->srcu); r = kvm_x86_ops.nested_ops->set_state(vcpu, user_kvm_nested_state, &kvm_state); srcu_read_unlock(&vcpu->kvm->srcu, idx); with vmx_set_nested_state() then copying the vmcs12 from user 2. KVM_GET_MSRS / KVM_SET_MSRS — arch/x86/kvm/x86.c case KVM_SET_MSRS: { int idx = srcu_read_lock(&vcpu->kvm->srcu); r = msr_io(vcpu, argp, do_set_msr, 0); srcu_read_unlock(&vcpu->kvm->srcu, idx); break; } argp goes in as a raw void __user *, and msr_io() does both user copies inside that section: if (copy_from_user(&msrs, user_msrs, sizeof(msrs))) 3. KVM_S390_MEM_OP kvm_s390_vm_mem_op_abs() has scoped_guard(srcu, &kvm->srcu) { [...] if (acc_mode == GACC_STORE && copy_from_user(tmpbuf, uaddr, mop->size)) return -EFAULT; 4. KVM_PRE_FAULT_MEMORY generic KVM holds srcu across a loop of kvm_arch_vcpu_pre_fault_memory() calls that fault in guest memory, 5. and last but not least,isnt vcpu_run also holding kvm->srcu when handling page faults? In the end the cycle seems userspace constructed. It deadlocks only if its own uffd handler is made to depend on a vcpu blocked on that same handler.The faulting side waits in TASK_KILLABLE (mm/userfaultfd.c:2673) — SIGKILL to the process releases the srcu section and lets the other thread finish. So we can get out of it. So I think this finding is wrong. Paolo?