From edd03602d97236e8fea13cd76886c576186aa307 Mon Sep 17 00:00:00 2001 From: Paul Mackerras Date: Mon, 28 Aug 2017 14:31:24 +1000 Subject: [PATCH] KVM: PPC: Book3S HV: Protect updates to spapr_tce_tables list Al Viro pointed out that while one thread of a process is executing in kvm_vm_ioctl_create_spapr_tce(), another thread could guess the file descriptor returned by anon_inode_getfd() and close() it before the first thread has added it to the kvm->arch.spapr_tce_tables list. That highlights a more general problem: there is no mutual exclusion between writers to the spapr_tce_tables list, leading to the possibility of the list becoming corrupted, which could cause a host kernel crash. To fix the mutual exclusion problem, we add a mutex_lock/unlock pair around the list_del_rce in kvm_spapr_tce_release(). Also, this moves the call to anon_inode_getfd() inside the region protected by the kvm->lock mutex, after we have done the check for a duplicate LIOBN. This means that if another thread does guess the file descriptor and closes it, its call to kvm_spapr_tce_release() will not do any harm because it will have to wait until the first thread has released kvm->lock. With this, there are no failure points in kvm_vm_ioctl_create_spapr_tce() after the call to anon_inode_getfd(). The other things that the second thread could do with the guessed file descriptor are to mmap it or to pass it as a parameter to a KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE ioctl on a KVM device fd. An mmap call won't cause any harm because kvm_spapr_tce_mmap() and kvm_spapr_tce_fault() don't access the spapr_tce_tables list or the kvmppc_spapr_tce_table.list field, and the fields that they do use have been properly initialized by the time of the anon_inode_getfd() call. The KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE ioctl calls kvm_spapr_tce_attach_iommu_group(), which scans the spapr_tce_tables list looking for the kvmppc_spapr_tce_table struct corresponding to the fd given as the parameter. Either it will find the new entry or it won't; if it doesn't, it just returns an error, and if it does, it will function normally. So, in each case there is no harmful effect. Reviewed-by: David Gibson Signed-off-by: Paul Mackerras --- arch/powerpc/kvm/book3s_64_vio.c | 21 ++++++++++----------- 1 file changed, 10 insertions(+), 11 deletions(-) diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c index 53766e2bc029..8f2da8bba737 100644 --- a/arch/powerpc/kvm/book3s_64_vio.c +++ b/arch/powerpc/kvm/book3s_64_vio.c @@ -265,8 +265,11 @@ static int kvm_spapr_tce_release(struct inode *inode, struct file *filp) { struct kvmppc_spapr_tce_table *stt = filp->private_data; struct kvmppc_spapr_tce_iommu_table *stit, *tmp; + struct kvm *kvm = stt->kvm; + mutex_lock(&kvm->lock); list_del_rcu(&stt->list); + mutex_unlock(&kvm->lock); list_for_each_entry_safe(stit, tmp, &stt->iommu_tables, next) { WARN_ON(!kref_read(&stit->kref)); @@ -298,7 +301,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm, unsigned long npages, size; int ret = -ENOMEM; int i; - int fd = -1; if (!args->size) return -EINVAL; @@ -328,11 +330,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm, goto fail; } - ret = fd = anon_inode_getfd("kvm-spapr-tce", &kvm_spapr_tce_fops, - stt, O_RDWR | O_CLOEXEC); - if (ret < 0) - goto fail; - mutex_lock(&kvm->lock); /* Check this LIOBN hasn't been previously allocated */ @@ -344,17 +341,19 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm, } } - if (!ret) { + if (!ret) + ret = anon_inode_getfd("kvm-spapr-tce", &kvm_spapr_tce_fops, + stt, O_RDWR | O_CLOEXEC); + + if (ret >= 0) { list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables); kvm_get_kvm(kvm); } mutex_unlock(&kvm->lock); - if (!ret) - return fd; - - put_unused_fd(fd); + if (ret >= 0) + return ret; fail: for (i = 0; i < npages; i++) -- 2.39.2