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 <[email protected]>
---
arch/powerpc/kvm/book3s_64_vio.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
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);
--
2.23.0
Hi Leonardo,
Leonardo Bras <[email protected]> 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 <[email protected]> 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,