| Message ID | 20191107170258.36379-2-leonardo@linux.ibm.com |
|---|---|
| State | Rejected |
| Headers | show |
| Series | Replace current->mm by kvm->mm on powerpc/kvm | expand |
Hi Leonardo, Leonardo Bras <leonardo@linux.ibm.com> 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. > 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); > @@ -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
On Tue, 2019-11-12 at 15:57 +1100, Michael Ellerman wrote: > Hi Leonardo, Hello Micheal, thanks for the feedback! > > Leonardo Bras <leonardo@linux.ibm.com> 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. > > > 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); > > > @@ -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. > So, there is no chance that between this function's kvm_get_kvm() and kvm_put_kvm(), another thread can decrease this reference counter? > 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 Kind regards
On Thu, 2019-11-14 at 15:43 -0300, Leonardo Bras wrote: > > 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. > > > > So, there is no chance that between this function's kvm_get_kvm() > and > kvm_put_kvm(), another thread can decrease this reference counter? I am probably missing something here, could you please help me understand that? > > 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 > > Kind regards Best regards,
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 @@ -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);
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). Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com> --- arch/powerpc/kvm/book3s_64_vio.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)