[PATCH v3 1/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

Janis Schoetterl-Glausch
2022-11-17T22:19:36Z

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 | 5 ++
arch/s390/kvm/gaccess.h | 3 ++
arch/s390/kvm/gaccess.c | 101 +++++++++++++++++++++++++++++++++++++++
arch/s390/kvm/kvm-s390.c | 35 +++++++++++++-
4 files changed, 142 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 0d5d4419139a..1f36be5493e6 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -588,6 +588,8 @@ 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_p; /* ignored if flag unset */
};
__u32 sida_offset; /* offset into the sida */
__u8 reserved[32]; /* ignored */
@@ -604,6 +606,9 @@ struct kvm_s390_mem_op {
#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)
+#define KVM_S390_MEMOP_F_CMPXCHG (1ULL << 3)
+/* Non program exception return codes (pgm codes are 16 bit) */
+#define KVM_S390_MEMOP_R_NO_XCHG ((1 << 16) + 0)

/* for KVM_INTERRUPT */
struct kvm_interrupt {
diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
index 9408d6cc8e2c..92a3b9fb31ec 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);
+
/**
* 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..be042865d8a1 100644
--- a/arch/s390/kvm/gaccess.c
+++ b/arch/s390/kvm/gaccess.c
@@ -1161,6 +1161,107 @@ 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_p: Pointer to old value. If the location at @gpa contains this value, the
+ * exchange will succeed. After calling cmpxchg_guest_abs_with_key() *@old
+ * 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.
+ *
+ * Atomically exchange the value at @gpa by @new, if it contains *@old.
+ * Honors storage keys.
+ *
+ * Return: * 0: successful exchange
+ * * 1: exchange unsuccessful
+ * * 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)
+ */
+int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,
+ __uint128_t *old_p, __uint128_t new,
+ u8 access_key)
+{
+ gfn_t gfn = gpa >> PAGE_SHIFT;
+ 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);
+ switch (len) {
+ case 1: {
+ u8 old;
+
+ ret = cmpxchg_user_key((u8 *)hva, &old, *old_p, new, access_key);
+ ret = ret < 0 ? ret : old != *old_p;
+ *old_p = old;
+ break;
+ }
+ case 2: {
+ u16 old;
+
+ ret = cmpxchg_user_key((u16 *)hva, &old, *old_p, new, access_key);
+ ret = ret < 0 ? ret : old != *old_p;
+ *old_p = old;
+ break;
+ }
+ case 4: {
+ u32 old;
+
+ ret = cmpxchg_user_key((u32 *)hva, &old, *old_p, new, access_key);
+ ret = ret < 0 ? ret : old != *old_p;
+ *old_p = old;
+ break;
+ }
+ case 8: {
+ u64 old;
+
+ ret = cmpxchg_user_key((u64 *)hva, &old, *old_p, new, access_key);
+ ret = ret < 0 ? ret : old != *old_p;
+ *old_p = old;
+ break;
+ }
+ case 16: {
+ __uint128_t old;
+
+ ret = cmpxchg_user_key((__uint128_t *)hva, &old, *old_p, new, access_key);
+ ret = ret < 0 ? ret : old != *old_p;
+ *old_p = old;
+ break;
+ }
+ default:
+ return -EINVAL;
+ }
+ 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 45d4b8182b07..2410b4044283 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -576,7 +576,6 @@ 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:
@@ -590,6 +589,14 @@ 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.
+ * The first extension doesn't use a flag, but pretend it does,
+ * this way that can be changed in the future.
+ */
+ r = 0x3;
+ break;
case KVM_CAP_NR_VCPUS:
case KVM_CAP_MAX_VCPUS:
case KVM_CAP_MAX_VCPU_ID:
@@ -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_p = (void __user *)mop->old_p;
+ union {
+ __uint128_t quad;
+ char raw[sizeof(__uint128_t)];
+ } old = { .quad = 0}, new = { .quad = 0 };
+ unsigned int off_in_quad = sizeof(__uint128_t) - 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 +2755,15 @@ 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) {
+ if (mop->size > sizeof(new))
+ return -EINVAL;
+ /* off_in_quad has been validated */
+ if (copy_from_user(&new.raw[off_in_quad], uaddr, mop->size))
+ return -EFAULT;
+ if (copy_from_user(&old.raw[off_in_quad], old_p, mop->size))
+ return -EFAULT;
+ }
if (!(mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)) {
tmpbuf = vmalloc(mop->size);
if (!tmpbuf)
@@ -2771,6 +2794,14 @@ 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_p, &old.raw[off_in_quad], mop->size))
+ r = -EFAULT;
+ }
} else {
if (copy_from_user(tmpbuf, uaddr, mop->size)) {
r = -EFAULT;
--
2.34.1