Re: [PATCH] KVM: s390: Fix lockdep issue in vm memop

[Christian Borntraeger]

Am 23.03.22 um 09:57 schrieb Janosch Frank:
> On 3/23/22 09:52, Janis Schoetterl-Glausch wrote:
> > On 3/23/22 08:58, Janosch Frank wrote:
> > > On 3/22/22 16:32, Janis Schoetterl-Glausch wrote:
> > > > Issuing a memop on a protected vm does not make sense,
> > >
> > > Issuing a vm memop on a protected vm...
> > >
> > > The cpu memop still makes sense, no?
> >
> > The vcpu memop does hold the vcpu->lock, so no lockdep issue.
> > If you issue a vcpu memop while enabling protected virtualization,
> > the memop might find that the vcpu is not protected, while other vcpus
> > might already be, but I don't think there's a way to create secure memory
> > concurrent with the memop.
>
> I just wanted you to make this a bit more specific since we now have vm and vcpu memops. vm memops don't make sense for pv guests but vcpu ones are needed to access the sida.

Right, I think changing the commit messages
- Issuing a memop on a protected vm does not make sense
+ Issuing a vm memop on a protected vm does not make sense

does make sense.

> > > > neither is the memory readable/writable, nor does it make sense to check
> > > > storage keys. This is why the ioctl will return -EINVAL when it detects
> > > > the vm to be protected. However, in order to ensure that the vm cannot
> > > > become protected during the memop, the kvm->lock would need to be taken
> > > > for the duration of the ioctl. This is also required because
> > > > kvm_s390_pv_is_protected asserts that the lock must be held.
> > > > Instead, don't try to prevent this. If user space enables secure
> > > > execution concurrently with a memop it must accecpt the possibility of
> > > > the memop failing.
> > > > Still check if the vm is currently protected, but without locking and
> > > > consider it a heuristic.
> > > >
> > > > Fixes: ef11c9463ae0 ("KVM: s390: Add vm IOCTL for key checked guest absolute memory access")
> > > > Signed-off-by: Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx>
> > >
> > > Makes sense to me.
> > >
> > > Reviewed-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
> > >
> > > > ---
> > > >    arch/s390/kvm/kvm-s390.c | 11 ++++++++++-
> > > >    1 file changed, 10 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > > > index ca96f84db2cc..53adbe86a68f 100644
> > > > --- a/arch/s390/kvm/kvm-s390.c
> > > > +++ b/arch/s390/kvm/kvm-s390.c
> > > > @@ -2385,7 +2385,16 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
> > > >            return -EINVAL;
> > > >        if (mop->size > MEM_OP_MAX_SIZE)
> > > >            return -E2BIG;
> > > > -    if (kvm_s390_pv_is_protected(kvm))
> > > > +    /*
> > > > +     * This is technically a heuristic only, if the kvm->lock is not
> > > > +     * taken, it is not guaranteed that the vm is/remains non-protected.
> > > > +     * This is ok from a kernel perspective, wrongdoing is detected
> > > > +     * on the access, -EFAULT is returned and the vm may crash the
> > > > +     * next time it accesses the memory in question.
> > > > +     * There is no sane usecase to do switching and a memop on two
> > > > +     * different CPUs at the same time.
> > > > +     */
> > > > +    if (kvm_s390_pv_get_handle(kvm))
> > > >            return -EINVAL;
> > > >        if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) {
> > > >            if (access_key_invalid(mop->key))
> > > >
> > > > base-commit: c9b8fecddb5bb4b67e351bbaeaa648a6f7456912