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AJvYcCVZBxqjMcUxPXte1iYacLC/pUENWCzmKJ+W8MST8+N9WPnFWMJEdjmC+Pnt46DktCh5sQURl61+5BVeS38q@vger.kernel.org X-Gm-Message-State: AOJu0YxtlMR4j7Ytk3dC39c362SIC7midg7SElBG00N6ohTo2mrK0woG eCpSXvTLP/0kiufmY1qKGyPL5ycG7W83g+cXf0MPif8sd8MwuWOBq+KbkwP30hGuisere06ZcKD bTg== X-Google-Smtp-Source: AGHT+IHrlpU6TSN8LFEDqLk4N7nyxIXqbtuZg3bjjLval5KFIlyghPAfN3T3Jm+iafOU8Hu0EqUexqT5TWc= X-Received: from pjbee11.prod.google.com ([2002:a17:90a:fc4b:b0:2fc:ccfe:368]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:1a89:b0:2fe:85f0:e115 with SMTP id 98e67ed59e1d1-30e2e65e6e3mr4418434a91.26.1747230769918; Wed, 14 May 2025 06:52:49 -0700 (PDT) Date: Wed, 14 May 2025 06:52:48 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250513163438.3942405-1-tabba@google.com> <20250513163438.3942405-9-tabba@google.com> Message-ID: Subject: Re: [PATCH v9 08/17] KVM: guest_memfd: Check that userspace_addr and fd+offset refer to same range From: Sean Christopherson To: Ackerley Tng Cc: Fuad Tabba , James Houghton , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, mail@maciej.szmigiero.name, david@redhat.com, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, peterx@redhat.com, pankaj.gupta@amd.com, ira.weiny@intel.com Content-Type: text/plain; charset="us-ascii" On Wed, May 14, 2025, Ackerley Tng wrote: > Sean Christopherson writes: > > On Wed, May 14, 2025, Fuad Tabba wrote: > >> On Tue, 13 May 2025 at 21:31, James Houghton wrote: > >> > > @@ -585,9 +611,14 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot, > >> > > offset + size > i_size_read(inode)) > >> > > goto err; > >> > > > >> > > - if (kvm_gmem_supports_shared(inode) && > >> > > - !kvm_arch_vm_supports_gmem_shared_mem(kvm)) > >> > > - goto err; > >> > > + if (kvm_gmem_supports_shared(inode)) { > >> > > + if (!kvm_arch_vm_supports_gmem_shared_mem(kvm)) > >> > > + goto err; > >> > > + > >> > > + if (slot->userspace_addr && > >> > > + !kvm_gmem_is_same_range(kvm, slot, file, offset)) > >> > > + goto err; > >> > > >> > This is very nit-picky, but I would rather this not be -EINVAL, maybe > >> > -EIO instead? Or maybe a pr_warn_once() and let the call proceed? > > > > Or just omit the check entirely. The check isn't binding (ba-dump, ching!), > > because the mapping/VMA can change the instant mmap_read_unlock() is called. > > > >> > The userspace_addr we got isn't invalid per se, we're just trying to > >> > give a hint to the user that their VMAs (or the userspace address they > >> > gave us) are messed up. I don't really like lumping this in with truly > >> > invalid arguments. > >> > >> I don't mind changing the return error, but I don't think that we > >> should have a kernel warning (pr_warn_once) for something userspace > >> can trigger. > > > > This isn't a WARN, e.g. won't trip panic_on_warn. In practice, it's not > > meaningfully different than pr_info(). That said, I agree that printing anything > > is a bad approach. > > > >> It's not an IO error either. I think that this is an invalid argument > >> (EINVAL). > > > > I agree with James, this isn't an invalid argument. Having the validity of an > > input hinge on the ordering between a KVM ioctl() and mmap() is quite odd. I > > know KVM arm64 does exactly this for KVM_SET_USER_MEMORY_REGION{,2}, but I don't > > love the semantics. And unlike that scenario, where e.g. MTE tags are verified > > again at fault-time, KVM won't re-check the VMA when accessing guest memory via > > the userspace mapping, e.g. through uaccess. > > > > Unless I'm forgetting something, I'm leaning toward omitting the check entirely. > > > > I'm good with dropping this patch. I might have misunderstood the conclusion > of the guest_memfd call. No, I don't think you misunderstood anything. It's just that sometimes opinions different when there's actual code, versus a verbal discussion. I.e. this sounds like a good idea, but when seeing the code and thinking through the effects, it's less appealing.