[PATCH v7 12/14] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

Janis Schoetterl-Glausch
2023-02-06T16:46:35Z

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
op. For now, support this op for absolute accesses only.

This op can be used, 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 | 8 +++
arch/s390/kvm/gaccess.h | 3 ++
arch/s390/kvm/gaccess.c | 109 +++++++++++++++++++++++++++++++++++++++
arch/s390/kvm/kvm-s390.c | 56 +++++++++++++++++++-
4 files changed, 175 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 55155e262646..d2f30463c133 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -583,6 +583,8 @@ struct kvm_s390_mem_op {
struct {
__u8 ar; /* the access register number */
__u8 key; /* access key, ignored if flag unset */
+ __u8 pad1[6]; /* ignored */
+ __u64 old_addr; /* ignored if cmpxchg flag unset */
};
__u32 sida_offset; /* offset into the sida */
__u8 reserved[32]; /* ignored */
@@ -595,11 +597,17 @@ struct kvm_s390_mem_op {
#define KVM_S390_MEMOP_SIDA_WRITE 3
#define KVM_S390_MEMOP_ABSOLUTE_READ 4
#define KVM_S390_MEMOP_ABSOLUTE_WRITE 5
+#define KVM_S390_MEMOP_ABSOLUTE_CMPXCHG 6
+
/* flags for kvm_s390_mem_op->flags */
#define KVM_S390_MEMOP_F_CHECK_ONLY (1ULL << 0)
#define KVM_S390_MEMOP_F_INJECT_EXCEPTION (1ULL << 1)
#define KVM_S390_MEMOP_F_SKEY_PROTECTION (1ULL << 2)

+/* flags specifying extension support via KVM_CAP_S390_MEM_OP_EXTENSION */
+#define KVM_S390_MEMOP_EXTENSION_CAP_BASE (1 << 0)
+#define KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG (1 << 1)
+
/* for KVM_INTERRUPT */
struct kvm_interrupt {
/* in */
diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
index 9408d6cc8e2c..b320d12aa049 100644
--- a/arch/s390/kvm/gaccess.h
+++ b/arch/s390/kvm/gaccess.h
@@ -206,6 +206,9 @@ int access_guest_with_key(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
void *data, unsigned long len, enum gacc_mode mode);

+int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len, __uint128_t *old,
+ __uint128_t new, u8 access_key, bool *success);
+
/**
* write_guest_with_key - copy data from kernel space to guest space
* @vcpu: virtual cpu
diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
index 0243b6e38d36..3eb85f254881 100644
--- a/arch/s390/kvm/gaccess.c
+++ b/arch/s390/kvm/gaccess.c
@@ -1161,6 +1161,115 @@ int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
return rc;
}

+/**
+ * cmpxchg_guest_abs_with_key() - Perform cmpxchg on guest absolute address.
+ * @kvm: Virtual machine instance.
+ * @gpa: Absolute guest address of the location to be changed.
+ * @len: Operand length of the cmpxchg, required: 1 <= len <= 16. Providing a
+ * non power of two will result in failure.
+ * @old_addr: Pointer to old value. If the location at @gpa contains this value,
+ * the exchange will succeed. After calling cmpxchg_guest_abs_with_key()
+ * *@old_addr contains the value at @gpa before the attempt to
+ * exchange the value.
+ * @new: The value to place at @gpa.
+ * @access_key: The access key to use for the guest access.
+ * @success: output value indicating if an exchange occurred.
+ *
+ * Atomically exchange the value at @gpa by @new, if it contains *@old.
+ * Honors storage keys.
+ *
+ * Return: * 0: successful exchange
+ * * >0: a program interruption code indicating the reason cmpxchg could
+ * not be attempted
+ * * -EINVAL: address misaligned or len not power of two
+ * * -EAGAIN: transient failure (len 1 or 2)
+ * * -EOPNOTSUPP: read-only memslot (should never occur)
+ */
+int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,
+ __uint128_t *old_addr, __uint128_t new,
+ u8 access_key, bool *success)
+{
+ gfn_t gfn = gpa_to_gfn(gpa);
+ struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+ bool writable;
+ hva_t hva;
+ int ret;
+
+ if (!IS_ALIGNED(gpa, len))
+ return -EINVAL;
+
+ hva = gfn_to_hva_memslot_prot(slot, gfn, &writable);
+ if (kvm_is_error_hva(hva))
+ return PGM_ADDRESSING;
+ /*
+ * Check if it's a read-only memslot, even though that cannot occur
+ * since those are unsupported.
+ * Don't try to actually handle that case.
+ */
+ if (!writable)
+ return -EOPNOTSUPP;
+
+ hva += offset_in_page(gpa);
+ /*
+ * The cmpxchg_user_key macro depends on the type of "old", so we need
+ * a case for each valid length and get some code duplication as long
+ * as we don't introduce a new macro.
+ */
+ switch (len) {
+ case 1: {
+ u8 old;
+
+ ret = cmpxchg_user_key((u8 __user *)hva, &old, *old_addr, new, access_key);
+ *success = !ret && old == *old_addr;
+ *old_addr = old;
+ break;
+ }
+ case 2: {
+ u16 old;
+
+ ret = cmpxchg_user_key((u16 __user *)hva, &old, *old_addr, new, access_key);
+ *success = !ret && old == *old_addr;
+ *old_addr = old;
+ break;
+ }
+ case 4: {
+ u32 old;
+
+ ret = cmpxchg_user_key((u32 __user *)hva, &old, *old_addr, new, access_key);
+ *success = !ret && old == *old_addr;
+ *old_addr = old;
+ break;
+ }
+ case 8: {
+ u64 old;
+
+ ret = cmpxchg_user_key((u64 __user *)hva, &old, *old_addr, new, access_key);
+ *success = !ret && old == *old_addr;
+ *old_addr = old;
+ break;
+ }
+ case 16: {
+ __uint128_t old;
+
+ ret = cmpxchg_user_key((__uint128_t __user *)hva, &old, *old_addr, new, access_key);
+ *success = !ret && old == *old_addr;
+ *old_addr = old;
+ break;
+ }
+ default:
+ return -EINVAL;
+ }
+ if (*success)
+ mark_page_dirty_in_slot(kvm, slot, gfn);
+ /*
+ * Assume that the fault is caused by protection, either key protection
+ * or user page write protection.
+ */
+ if (ret == -EFAULT)
+ ret = PGM_PROTECTION;
+ return ret;
+}
+
/**
* guest_translate_address_with_key - translate guest logical into guest absolute address
* @vcpu: virtual cpu
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 8a74b7b62ecf..5b77269e6536 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -584,7 +584,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_S390_VCPU_RESETS:
case KVM_CAP_SET_GUEST_DEBUG:
case KVM_CAP_S390_DIAG318:
- case KVM_CAP_S390_MEM_OP_EXTENSION:
r = 1;
break;
case KVM_CAP_SET_GUEST_DEBUG2:
@@ -598,6 +597,15 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_S390_MEM_OP:
r = MEM_OP_MAX_SIZE;
break;
+ case KVM_CAP_S390_MEM_OP_EXTENSION:
+ /*
+ * Flag bits indicating which extensions are supported.
+ * If r > 0, the base extension must also be supported/indicated,
+ * in order to maintain backwards compatibility.
+ */
+ r = KVM_S390_MEMOP_EXTENSION_CAP_BASE |
+ KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG;
+ break;
case KVM_CAP_NR_VCPUS:
case KVM_CAP_MAX_VCPUS:
case KVM_CAP_MAX_VCPU_ID:
@@ -2832,6 +2840,50 @@ static int kvm_s390_vm_mem_op_abs(struct kvm *kvm, struct kvm_s390_mem_op *mop)
return r;
}

+static int kvm_s390_vm_mem_op_cmpxchg(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;
+ int r, srcu_idx;
+ bool success;
+
+ r = mem_op_validate_common(mop, KVM_S390_MEMOP_F_SKEY_PROTECTION);
+ if (r)
+ return r;
+ /*
+ * 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;
+ 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;
+
+ srcu_idx = srcu_read_lock(&kvm->srcu);
+
+ if (kvm_is_error_gpa(kvm, mop->gaddr)) {
+ r = PGM_ADDRESSING;
+ goto out_unlock;
+ }
+
+ r = cmpxchg_guest_abs_with_key(kvm, mop->gaddr, mop->size, &old.quad,
+ new.quad, mop->key, &success);
+ if (!success && copy_to_user(old_addr, &old.raw[off_in_quad], mop->size))
+ r = -EFAULT;
+
+out_unlock:
+ srcu_read_unlock(&kvm->srcu, srcu_idx);
+ return r;
+}
+
static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
{
/*
@@ -2850,6 +2902,8 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
case KVM_S390_MEMOP_ABSOLUTE_READ:
case KVM_S390_MEMOP_ABSOLUTE_WRITE:
return kvm_s390_vm_mem_op_abs(kvm, mop);
+ case KVM_S390_MEMOP_ABSOLUTE_CMPXCHG:
+ return kvm_s390_vm_mem_op_cmpxchg(kvm, mop);
default:
return -EINVAL;
}
--
2.37.2