Re: [PATCH v4 1/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

On Wed, 2022-12-14 at 10:19 +0100, Thomas Huth wrote:
> On 13/12/2022 17.53, Janis Schoetterl-Glausch wrote:
> > User space can use the MEM_OP ioctl to make storage key checked reads
> > and writes to the guest, however, it has no way of performing atomic,
> > key checked, accesses to the guest.
> > Extend the MEM_OP ioctl in order to allow for this, by adding a cmpxchg
> > mode. For now, support this mode for absolute accesses only.
> > 
> > This mode can be use, for example, to set the device-state-change
> > indicator and the adapter-local-summary indicator atomically.
> > 
> > Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
> > ---
> >   include/uapi/linux/kvm.h |   7 +++
> >   arch/s390/kvm/gaccess.h  |   3 ++
> >   arch/s390/kvm/gaccess.c  | 102 +++++++++++++++++++++++++++++++++++++++
> >   arch/s390/kvm/kvm-s390.c |  39 ++++++++++++++-
> >   4 files changed, 149 insertions(+), 2 deletions(-)
> > 
[...]
> > 
> > @@ -2714,12 +2721,19 @@ static bool access_key_invalid(u8 access_key)
> >   static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
> >   {
> >   	void __user *uaddr = (void __user *)mop->buf;
> > +	void __user *old_addr = (void __user *)mop->old_addr;
> > +	union {
> > +		__uint128_t quad;
> > +		char raw[sizeof(__uint128_t)];
> > +	} old = { .quad = 0}, new = { .quad = 0 };
> > +	unsigned int off_in_quad = sizeof(new) - mop->size;
> >   	u64 supported_flags;
> >   	void *tmpbuf = NULL;
> >   	int r, srcu_idx;
> >   
> >   	supported_flags = KVM_S390_MEMOP_F_SKEY_PROTECTION
> > -			  | KVM_S390_MEMOP_F_CHECK_ONLY;
> > +			  | KVM_S390_MEMOP_F_CHECK_ONLY
> > +			  | KVM_S390_MEMOP_F_CMPXCHG;
> >   	if (mop->flags & ~supported_flags || !mop->size)
> >   		return -EINVAL;
> >   	if (mop->size > MEM_OP_MAX_SIZE)
> > @@ -2741,6 +2755,19 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
> >   	} else {
> >   		mop->key = 0;
> >   	}
> > +	if (mop->flags & KVM_S390_MEMOP_F_CMPXCHG) {
> > +		/*
> > +		 * This validates off_in_quad. Checking that size is a power
> > +		 * of two is not necessary, as cmpxchg_guest_abs_with_key
> > +		 * takes care of that
> > +		 */
> > +		if (mop->size > sizeof(new))
> > +			return -EINVAL;
> 
> I'd maybe add a check for mop->op == KVM_S390_MEMOP_ABSOLUTE_WRITE here, 
> since calling the _READ function with the F_CMPXCHG flag set does not make 
> too much sense.

Good point.
> 
> Anyway, patch looks good to me, so with or without that additional check:
> Reviewed-by: Thomas Huth <thuth@redhat.com>

Thanks!
> 
> > +		if (copy_from_user(&new.raw[off_in_quad], uaddr, mop->size))
> > +			return -EFAULT;
> > +		if (copy_from_user(&old.raw[off_in_quad], old_addr, mop->size))
> > +			return -EFAULT;
> > +	}
> >   	if (!(mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)) {
> >   		tmpbuf = vmalloc(mop->size);
> >   		if (!tmpbuf)
> > @@ -2771,6 +2798,14 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
> >   	case KVM_S390_MEMOP_ABSOLUTE_WRITE: {
> >   		if (mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY) {
> >   			r = check_gpa_range(kvm, mop->gaddr, mop->size, GACC_STORE, mop->key);
> > +		} else if (mop->flags & KVM_S390_MEMOP_F_CMPXCHG) {
> > +			r = cmpxchg_guest_abs_with_key(kvm, mop->gaddr, mop->size,
> > +						       &old.quad, new.quad, mop->key);
> > +			if (r == 1) {
> > +				r = KVM_S390_MEMOP_R_NO_XCHG;
> > +				if (copy_to_user(old_addr, &old.raw[off_in_quad], mop->size))
> > +					r = -EFAULT;
> > +			}
> >   		} else {
> >   			if (copy_from_user(tmpbuf, uaddr, mop->size)) {
> >   				r = -EFAULT;
>