Re: [PATCH v2 05/11] KVM: s390: Add optional storage key checking to MEMOP IOCTL

From: Janis Schoetterl-Glausch
Date: Wed Feb 09 2022 - 07:07:58 EST

On Wed, 2022-02-09 at 11:48 +0100, Christian Borntraeger wrote:
>
> Am 09.02.22 um 11:39 schrieb Janis Schoetterl-Glausch:
> > On 2/9/22 11:08, Christian Borntraeger wrote:
> > >
> > > Am 09.02.22 um 11:01 schrieb Janis Schoetterl-Glausch:
> > > > On Wed, 2022-02-09 at 10:08 +0100, Christian Borntraeger wrote:
> > > > > Am 09.02.22 um 09:49 schrieb Janis Schoetterl-Glausch:
> > > > > > On 2/9/22 08:34, Christian Borntraeger wrote:
> > > > > > > Am 07.02.22 um 17:59 schrieb Janis Schoetterl-Glausch:
> > > > > > > > User space needs a mechanism to perform key checked accesses when
> > > > > > > > emulating instructions.
> > > > > > > >
> > > > > > > > The key can be passed as an additional argument.
> > > > > > > > Having an additional argument is flexible, as user space can
> > > > > > > > pass the guest PSW's key, in order to make an access the same way the
> > > > > > > > CPU would, or pass another key if necessary.
> > > > > > > >
> > > > > > > > Signed-off-by: Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx>
> > > > > > > > Acked-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
> > > > > > > > Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> > > > > > > > ---
> > > > > > > > arch/s390/kvm/kvm-s390.c | 49 +++++++++++++++++++++++++++++++---------
> > > > > > > > include/uapi/linux/kvm.h | 8 +++++--
> > > > > > > > 2 files changed, 44 insertions(+), 13 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > > > > > > > index cf347e1a4f17..71e61fb3f0d9 100644
> > > > > > > > --- a/arch/s390/kvm/kvm-s390.c
> > > > > > > > +++ b/arch/s390/kvm/kvm-s390.c
> > > > > > > > @@ -32,6 +32,7 @@
> > > > > > > > #include <linux/sched/signal.h>
> > > > > > > > #include <linux/string.h>
> > > > > > > > #include <linux/pgtable.h>
> > > > > > > > +#include <linux/bitfield.h>
> > > > > > > > #include <asm/asm-offsets.h>
> > > > > > > > #include <asm/lowcore.h>
> > > > > > > > @@ -2359,6 +2360,11 @@ static int kvm_s390_handle_pv(struct kvm *kvm, struct kvm_pv_cmd *cmd)
> > > > > > > > return r;
> > > > > > > > }
> > > > > > > > +static bool access_key_invalid(u8 access_key)
> > > > > > > > +{
> > > > > > > > + return access_key > 0xf;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > long kvm_arch_vm_ioctl(struct file *filp,
> > > > > > > > unsigned int ioctl, unsigned long arg)
> > > > > > > > {
> > > > > > > > @@ -4687,34 +4693,54 @@ static long kvm_s390_guest_mem_op(struct kvm_vcpu *vcpu,
> > > > > > > > struct kvm_s390_mem_op *mop)
> > > > > > > > {
> > > > > > > > void __user *uaddr = (void __user *)mop->buf;
> > > > > > > > + u8 access_key = 0, ar = 0;
> > > > > > > > void *tmpbuf = NULL;
> > > > > > > > + bool check_reserved;
> > > > > > > > int r = 0;
> > > > > > > > const u64 supported_flags = KVM_S390_MEMOP_F_INJECT_EXCEPTION
> > > > > > > > - | KVM_S390_MEMOP_F_CHECK_ONLY;
> > > > > > > > + | KVM_S390_MEMOP_F_CHECK_ONLY
> > > > > > > > + | KVM_S390_MEMOP_F_SKEY_PROTECTION;
> > > > > > > > - if (mop->flags & ~supported_flags || mop->ar >= NUM_ACRS || !mop->size)
> > > > > > > > + if (mop->flags & ~supported_flags || !mop->size)
> > > > > > > > return -EINVAL;
> > > > > > > > -
> > > > > > > > if (mop->size > MEM_OP_MAX_SIZE)
> > > > > > > > return -E2BIG;
> > > > > > > > -
> > > > > > > > if (kvm_s390_pv_cpu_is_protected(vcpu))
> > > > > > > > return -EINVAL;
> > > > > > > > -
> > > > > > > > if (!(mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)) {
> > > > > > > > tmpbuf = vmalloc(mop->size);
> > > > > > > > if (!tmpbuf)
> > > > > > > > return -ENOMEM;
> > > > > > > > }
> > > > > > > > + ar = mop->ar;
> > > > > > > > + mop->ar = 0;
> > > > > > >
> > > > > > > Why this assignment to 0?
> > > > > >
> > > > > > It's so the check of reserved below works like that, they're all part of the anonymous union.
> > > > >
> > > > > Ah, I see. This is ugly :-)
> > > >
> > > > Yes :)
> > > > > > > > + if (ar >= NUM_ACRS)
> > > > > > > > + return -EINVAL;
> > > > > > > > + if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) {
> > > > > > > > + access_key = mop->key;
> > > > > > > > + mop->key = 0;
> > > > > > >
> > > > > > > and this? I think we can leave mop unchanged.
> > > > > > >
> > > > > > > In fact, why do we add the ar and access_key variable?
> > > > > > > This breaks the check from above (if (mop->flags & ~supported_flags || mop->ar >= NUM_ACRS || !mop->size)) into two checks
> > > > > > > and it will create a memleak for tmpbuf.
> > > > > >
> > > > > > I can move the allocation down, goto out or get rid of the reserved check and keep everything as before.
> > > > > > First is simpler, but second makes handling that case more explicit and might help in the future.
> > > > >
> > > > > Maybe add a reserved_02 field in the anon struct and check this for being zero and get rid of the local variables?
> > > >
> > > > I think that would require us adding new fields in the struct by putting them in a union with reserved_02 and so on,
> > > > which could get rather messy.
> > >
> > > I think it is fine to rename reserved_02. Maybe rename that to dont_use_02 ?
> >
> > I don't know what kind of stability guarantees we give here, since it can only happen when recompiling with
> > a new header. dont_use is a lot better than reserved here, after all we tell user space to set
> > reserved bytes to 0, using reserved_02 to do that would be quite handy and therefore likely.
> >
> > The question is also what semantic we want for the check.
> > The way it works right now, user space also needs to set unused fields to 0, e.g. key if the flag is not set.
> > At least this is the case for the vm memop, the vcpu memop cannot do that because of backward compatibility.
>
> As an alternative just remove the check for reserved == 0 and do that later on as an add-on patch?

That would kinda defeat the purpose of the check, since misbehaving user space programs would
get an error then but not now.