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Wed, 23 Mar 2022 09:30:32 +0000 (GMT) Message-ID: <968319ed-ae4b-02fe-41c4-06799e940d94@linux.ibm.com> Date: Wed, 23 Mar 2022 10:30:32 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.0 Subject: Re: [PATCH] KVM: s390: Fix lockdep issue in vm memop Content-Language: en-US To: Janosch Frank , Janis Schoetterl-Glausch , Claudio Imbrenda , Heiko Carstens , Vasily Gorbik , Alexander Gordeev Cc: David Hildenbrand , Sven Schnelle , kvm@vger.kernel.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org References: <20220322153204.2637400-1-scgl@linux.ibm.com> <44618f05-9aee-5aa5-b036-dd838285b26f@linux.ibm.com> <95c28949-8732-8812-c255-79467dafb5c8@linux.ibm.com> <7bcd8720-1c92-4e14-0c93-51d604f017a4@linux.ibm.com> From: Christian Borntraeger In-Reply-To: <7bcd8720-1c92-4e14-0c93-51d604f017a4@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: AHC1a6CJhtm_B1XSyEQz9Ea4w891V2ge X-Proofpoint-ORIG-GUID: ZMvIBKup2d96Qntgs3wUx-xIBoeG-kz5 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.850,Hydra:6.0.425,FMLib:17.11.64.514 definitions=2022-03-23_05,2022-03-22_01,2022-02-23_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 clxscore=1015 priorityscore=1501 lowpriorityscore=0 phishscore=0 spamscore=0 bulkscore=0 adultscore=0 mlxscore=0 impostorscore=0 malwarescore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2202240000 definitions=main-2203230055 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org Am 23.03.22 um 09:57 schrieb Janosch Frank: > On 3/23/22 09:52, Janis Schoetterl-Glausch wrote: >> On 3/23/22 08:58, Janosch Frank wrote: >>> On 3/22/22 16:32, Janis Schoetterl-Glausch wrote: >>>> Issuing a memop on a protected vm does not make sense, >>> >>> Issuing a vm memop on a protected vm... >>> >>> The cpu memop still makes sense, no? >> >> The vcpu memop does hold the vcpu->lock, so no lockdep issue. >> If you issue a vcpu memop while enabling protected virtualization, >> the memop might find that the vcpu is not protected, while other vcpus >> might already be, but I don't think there's a way to create secure memory >> concurrent with the memop. > > I just wanted you to make this a bit more specific since we now have vm and vcpu memops. vm memops don't make sense for pv guests but vcpu ones are needed to access the sida. Right, I think changing the commit messages - Issuing a memop on a protected vm does not make sense + Issuing a vm memop on a protected vm does not make sense does make sense. > >>> >>>> neither is the memory readable/writable, nor does it make sense to check >>>> storage keys. This is why the ioctl will return -EINVAL when it detects >>>> the vm to be protected. However, in order to ensure that the vm cannot >>>> become protected during the memop, the kvm->lock would need to be taken >>>> for the duration of the ioctl. This is also required because >>>> kvm_s390_pv_is_protected asserts that the lock must be held. >>>> Instead, don't try to prevent this. If user space enables secure >>>> execution concurrently with a memop it must accecpt the possibility of >>>> the memop failing. >>>> Still check if the vm is currently protected, but without locking and >>>> consider it a heuristic. >>>> >>>> Fixes: ef11c9463ae0 ("KVM: s390: Add vm IOCTL for key checked guest absolute memory access") >>>> Signed-off-by: Janis Schoetterl-Glausch >>> >>> Makes sense to me. >>> >>> Reviewed-by: Janosch Frank >>> >>>> --- >>>>    arch/s390/kvm/kvm-s390.c | 11 ++++++++++- >>>>    1 file changed, 10 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c >>>> index ca96f84db2cc..53adbe86a68f 100644 >>>> --- a/arch/s390/kvm/kvm-s390.c >>>> +++ b/arch/s390/kvm/kvm-s390.c >>>> @@ -2385,7 +2385,16 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop) >>>>            return -EINVAL; >>>>        if (mop->size > MEM_OP_MAX_SIZE) >>>>            return -E2BIG; >>>> -    if (kvm_s390_pv_is_protected(kvm)) >>>> +    /* >>>> +     * This is technically a heuristic only, if the kvm->lock is not >>>> +     * taken, it is not guaranteed that the vm is/remains non-protected. >>>> +     * This is ok from a kernel perspective, wrongdoing is detected >>>> +     * on the access, -EFAULT is returned and the vm may crash the >>>> +     * next time it accesses the memory in question. >>>> +     * There is no sane usecase to do switching and a memop on two >>>> +     * different CPUs at the same time. >>>> +     */ >>>> +    if (kvm_s390_pv_get_handle(kvm)) >>>>            return -EINVAL; >>>>        if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) { >>>>            if (access_key_invalid(mop->key)) >>>> >>>> base-commit: c9b8fecddb5bb4b67e351bbaeaa648a6f7456912 >>> >> >