Re: [PATCH v2 1/4] powerpc/kvm/book3s: Fixes possible 'use after release' of kvm
From: Michael Ellerman <mpe@ellerman.id.au>
Date: 2019-11-12 04:57:45
Also in:
lkml
Hi Leonardo, Leonardo Bras [off-list ref] writes:
Fixes a possible 'use after free' of kvm variable in kvm_vm_ioctl_create_spapr_tce, where it does a mutex_unlock(&kvm->lock) after a kvm_put_kvm(kvm).
There is no potential for an actual use after free here AFAICS.
quoted hunk ↗ jump to hunk
diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c index 5834db0a54c6..a402ead833b6 100644 --- a/arch/powerpc/kvm/book3s_64_vio.c +++ b/arch/powerpc/kvm/book3s_64_vio.c
The preceeding context is:
mutex_lock(&kvm->lock);
/* Check this LIOBN hasn't been previously allocated */
ret = 0;
list_for_each_entry(siter, &kvm->arch.spapr_tce_tables, list) {
if (siter->liobn == args->liobn) {
ret = -EBUSY;
break;
}
}
kvm_get_kvm(kvm);
if (!ret)
ret = anon_inode_getfd("kvm-spapr-tce", &kvm_spapr_tce_fops,
stt, O_RDWR | O_CLOEXEC);
quoted hunk ↗ jump to hunk
@@ -316,14 +316,13 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm, if (ret >= 0) list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables); - else - kvm_put_kvm(kvm); mutex_unlock(&kvm->lock); if (ret >= 0) return ret; + kvm_put_kvm(kvm); kfree(stt); fail_acct: account_locked_vm(current->mm, kvmppc_stt_pages(npages), false);
If the kvm_put_kvm() you've moved actually caused the last reference to be dropped that would mean that our caller had passed us a kvm struct without holding a reference to it, and that would be a bug in our caller. Or put another way, it would mean the mutex_lock() above could already be operating on a freed kvm struct. The kvm_get_kvm() prior to the anon_inode_getfd() is to account for the reference that's held by the `stt` struct, and dropped in kvm_spapr_tce_release(). So although this patch isn't wrong, the explanation is not accurate. cheers