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AJvYcCVEY9Hu+zM2ZxeTyQaHrl0eSaa4PHWvC7r+pPF+wp6rQb/F7BLLPngezrWHnnqoTvXp3Ec=@vger.kernel.org X-Gm-Message-State: AOJu0YwlEMcgD9M45guRZaLyDkDBGv0Bt4fjU20zGBhGQVsxvRxsUB1X vCzp985o+lIdvZbmHpOetq0feoumClNrd9SFe16JY29SwPtSUpIgIip3oaHJ4Ew+re/C3UbRlcy YXg== X-Google-Smtp-Source: AGHT+IEBESmhDh1kZA9q3dK9bozi69K8VuHlCLiyf6hF2yMudSAXgRJciMpW68u3/8oFdg4WPhCaXZrhSmQ= X-Received: from pjbpb18.prod.google.com ([2002:a17:90b:3c12:b0:2f5:63a:4513]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:58cf:b0:2fa:f8d:65de with SMTP id 98e67ed59e1d1-30e2e62a98emr4631512a91.22.1747229568217; Wed, 14 May 2025 06:32:48 -0700 (PDT) Date: Wed, 14 May 2025 06:32:46 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@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: Fuad Tabba Cc: 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, ackerleytng@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="utf-8" Content-Transfer-Encoding: quoted-printable On Wed, May 14, 2025, Fuad Tabba wrote: > On Tue, 13 May 2025 at 21:31, James Houghton wrot= e: > > > > On Tue, May 13, 2025 at 9:34=E2=80=AFAM Fuad Tabba w= rote: > > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > > > index 8e6d1866b55e..2f499021df66 100644 > > > --- a/virt/kvm/guest_memfd.c > > > +++ b/virt/kvm/guest_memfd.c > > > @@ -556,6 +556,32 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_= create_guest_memfd *args) > > > return __kvm_gmem_create(kvm, size, flags); > > > } > > > > > > +static bool kvm_gmem_is_same_range(struct kvm *kvm, > > > + struct kvm_memory_slot *slot, > > > + struct file *file, loff_t offset) > > > +{ > > > + struct mm_struct *mm =3D kvm->mm; > > > + loff_t userspace_addr_offset; > > > + struct vm_area_struct *vma; > > > + bool ret =3D false; > > > + > > > + mmap_read_lock(mm); > > > + > > > + vma =3D vma_lookup(mm, slot->userspace_addr); > > > + if (!vma) > > > + goto out; > > > + > > > + if (vma->vm_file !=3D file) > > > + goto out; > > > + > > > + userspace_addr_offset =3D slot->userspace_addr - vma->vm_star= t; > > > + ret =3D userspace_addr_offset + (vma->vm_pgoff << PAGE_SHIFT)= =3D=3D offset; > > > +out: > > > + mmap_read_unlock(mm); > > > + > > > + return ret; > > > +} > > > + > > > int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot, > > > unsigned int fd, loff_t offset) > > > { > > > @@ -585,9 +611,14 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_me= mory_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. >=20 > 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 an= ything 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 veri= fied 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 enti= rely. > That said, other than opposing the idea of pr_warn, I am happy to change = it.