[PATCH 15/18] KVM: Introduce RWX memory attributes

Nicolas Saenz Julienne
2024-06-09T15:58:22Z

Declare memory attributes to map memory regions as non-readable,
non-writable, and/or non-executable.

The attributes are negated for the following reasons:
- Setting a 0 memory attribute (attr->attributes == 0) shouldn't
introduce any access restrictions. For example, when moving from
private to shared mappings in context of confidential computing.
- In practice, with negated attributes, a non-private RWX memory
attribute is analogous to a delete operation. It's a nice outcome, as
it forces remapping the region with huge-pages, doing the right thing
for use-cases that have short-lived access restricted regions like
Hyper-V's VSM.
- A non-negated version of the flags has no way of expressing
non-access mapping (NR/NW/NX) without having to introduce an extra
flag (since 0 isn't available).

Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
---
Documentation/virt/kvm/api.rst | 14 +++++++++++---
include/linux/kvm_host.h | 22 +++++++++++++++++++++-
include/uapi/linux/kvm.h | 3 +++
virt/kvm/kvm_main.c | 32 +++++++++++++++++++++++++++++---
4 files changed, 64 insertions(+), 7 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 18ddea9c4c58a..6d3bc5092ea63 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6313,15 +6313,23 @@ of guest physical memory.
__u64 flags;
};

+ #define KVM_MEMORY_ATTRIBUTE_NR (1ULL << 0)
+ #define KVM_MEMORY_ATTRIBUTE_NW (1ULL << 1)
+ #define KVM_MEMORY_ATTRIBUTE_NX (1ULL << 2)
#define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)

The address and size must be page aligned. The supported attributes can be
retrieved via ioctl(KVM_CHECK_EXTENSION) on KVM_CAP_MEMORY_ATTRIBUTES. If
executed on a VM, KVM_CAP_MEMORY_ATTRIBUTES precisely returns the attributes
supported by that VM. If executed at system scope, KVM_CAP_MEMORY_ATTRIBUTES
-returns all attributes supported by KVM. The only attribute defined at this
-time is KVM_MEMORY_ATTRIBUTE_PRIVATE, which marks the associated gfn as being
-guest private memory.
+returns all attributes supported by KVM. The attribute defined at this
+time are:
+
+ - KVM_MEMORY_ATTRIBUTE_NR/NW/NX - Respectively marks the memory region as
+ non-read, non-write and/or non-exec. Note that write-only, exec-only and
+ write-exec mappings are not supported.
+ - KVM_MEMORY_ATTRIBUTE_PRIVATE - Which marks the associated gfn as being guest
+ private memory.

Note, there is no "get" API. Userspace is responsible for explicitly tracking
the state of a gfn/page as needed.
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9250bf1c4db15..85378345e8e77 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2411,6 +2411,21 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
}

+static inline bool kvm_mem_attributes_may_read(u64 attrs)
+{
+ return !(attrs & KVM_MEMORY_ATTRIBUTE_NR);
+}
+
+static inline bool kvm_mem_attributes_may_write(u64 attrs)
+{
+ return !(attrs & KVM_MEMORY_ATTRIBUTE_NW);
+}
+
+static inline bool kvm_mem_attributes_may_exec(u64 attrs)
+{
+ return !(attrs & KVM_MEMORY_ATTRIBUTE_NX);
+}
+
#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
{
@@ -2423,7 +2438,7 @@ bool kvm_arch_pre_set_memory_attributes(struct kvm *kvm,
struct kvm_gfn_range *range);
bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
struct kvm_gfn_range *range);
-
+bool kvm_mem_attributes_valid(struct kvm *kvm, unsigned long attrs);
static inline bool kvm_memory_attributes_in_use(struct kvm *kvm)
{
return !xa_empty(&kvm->mem_attr_array);
@@ -2435,6 +2450,11 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE;
}
#else
+static inline bool kvm_mem_attributes_valid(struct kvm *kvm,
+ unsigned long attrs)
+{
+ return false;
+}
static inline bool kvm_memory_attributes_in_use(struct kvm *kvm)
{
return false;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 516d39910f9ab..26d4477dae8c6 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1550,6 +1550,9 @@ struct kvm_memory_attributes {
__u64 flags;
};

+#define KVM_MEMORY_ATTRIBUTE_NR (1ULL << 0)
+#define KVM_MEMORY_ATTRIBUTE_NW (1ULL << 1)
+#define KVM_MEMORY_ATTRIBUTE_NX (1ULL << 2)
#define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)

#define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 63c4b6739edee..bd27fc01e9715 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2430,10 +2430,14 @@ bool kvm_range_has_memory_attributes(struct kvm *kvm, gfn_t start, gfn_t end,

static u64 kvm_supported_mem_attributes(struct kvm *kvm)
{
+ u64 supported_attrs = KVM_MEMORY_ATTRIBUTE_NR |
+ KVM_MEMORY_ATTRIBUTE_NW |
+ KVM_MEMORY_ATTRIBUTE_NX;
+
if (!kvm || kvm_arch_has_private_mem(kvm))
- return KVM_MEMORY_ATTRIBUTE_PRIVATE;
+ supported_attrs |= KVM_MEMORY_ATTRIBUTE_PRIVATE;

- return 0;
+ return supported_attrs;
}

static __always_inline void kvm_handle_gfn_range(struct kvm *kvm,
@@ -2557,6 +2561,28 @@ static int kvm_vm_set_mem_attributes(struct kvm *kvm, gfn_t start, gfn_t end,

return r;
}
+
+bool kvm_mem_attributes_valid(struct kvm *kvm, unsigned long attrs)
+{
+ bool may_read = kvm_mem_attributes_may_read(attrs);
+ bool may_write = kvm_mem_attributes_may_write(attrs);
+ bool may_exec = kvm_mem_attributes_may_exec(attrs);
+ bool priv = attrs & KVM_MEMORY_ATTRIBUTE_PRIVATE;
+
+ if (attrs & ~kvm_supported_mem_attributes(kvm))
+ return false;
+
+ /* Private memory and access permissions are incompatible */
+ if (priv && (!may_read || !may_write || !may_exec))
+ return false;
+
+ /* Write and exec mappings require read access */
+ if ((may_write || may_exec) && !may_read)
+ return false;
+
+ return true;
+}
+
static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
struct kvm_memory_attributes *attrs)
{
@@ -2565,7 +2591,7 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
/* flags is currently not used. */
if (attrs->flags)
return -EINVAL;
- if (attrs->attributes & ~kvm_supported_mem_attributes(kvm))
+ if (!kvm_mem_attributes_valid(kvm, attrs->attributes))
return -EINVAL;
if (attrs->size == 0 || attrs->address + attrs->size < attrs->address)
return -EINVAL;
--
2.40.1