Re: [RFC PATCH v11 08/29] KVM: Introduce per-page memory attributes

From: Isaku Yamahata
Date: Wed Aug 02 2023 - 16:31:28 EST

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On Tue, Jul 18, 2023 at 04:44:51PM -0700,
Sean Christopherson <seanjc@xxxxxxxxxx> wrote:

> From: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
>
> In confidential computing usages, whether a page is private or shared is
> necessary information for KVM to perform operations like page fault
> handling, page zapping etc. There are other potential use cases for
> per-page memory attributes, e.g. to make memory read-only (or no-exec,
> or exec-only, etc.) without having to modify memslots.
>
> Introduce two ioctls (advertised by KVM_CAP_MEMORY_ATTRIBUTES) to allow
> userspace to operate on the per-page memory attributes.
> - KVM_SET_MEMORY_ATTRIBUTES to set the per-page memory attributes to
> a guest memory range.
> - KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES to return the KVM supported
> memory attributes.
>
> Use an xarray to store the per-page attributes internally, with a naive,
> not fully optimized implementation, i.e. prioritize correctness over
> performance for the initial implementation.
>
> Because setting memory attributes is roughly analogous to mprotect() on
> memory that is mapped into the guest, zap existing mappings prior to
> updating the memory attributes. Opportunistically provide an arch hook
> for the post-set path (needed to complete invalidation anyways) in
> anticipation of x86 needing the hook to update metadata related to
> determining whether or not a given gfn can be backed with various sizes
> of hugepages.
>
> It's possible that future usages may not require an invalidation, e.g.
> if KVM ends up supporting RWX protections and userspace grants _more_
> protections, but again opt for simplicity and punt optimizations to
> if/when they are needed.
>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/Y2WB48kD0J4VGynX@xxxxxxxxxx
> Cc: Fuad Tabba <tabba@xxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> Documentation/virt/kvm/api.rst | 60 ++++++++++++
> include/linux/kvm_host.h | 14 +++
> include/uapi/linux/kvm.h | 14 +++
> virt/kvm/Kconfig | 4 +
> virt/kvm/kvm_main.c | 170 +++++++++++++++++++++++++++++++++
> 5 files changed, 262 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 34d4ce66e0c8..0ca8561775ac 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6068,6 +6068,56 @@ writes to the CNTVCT_EL0 and CNTPCT_EL0 registers using the SET_ONE_REG
> interface. No error will be returned, but the resulting offset will not be
> applied.
>
> +4.139 KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES
> +-----------------------------------------
> +
> +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> +:Architectures: x86
> +:Type: vm ioctl
> +:Parameters: u64 memory attributes bitmask(out)
> +:Returns: 0 on success, <0 on error
> +
> +Returns supported memory attributes bitmask. Supported memory attributes will
> +have the corresponding bits set in u64 memory attributes bitmask.
> +
> +The following memory attributes are defined::
> +
> + #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)
> +
> +4.140 KVM_SET_MEMORY_ATTRIBUTES
> +-----------------------------------------
> +
> +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> +:Architectures: x86
> +:Type: vm ioctl
> +:Parameters: struct kvm_memory_attributes(in/out)
> +:Returns: 0 on success, <0 on error
> +
> +Sets memory attributes for pages in a guest memory range. Parameters are
> +specified via the following structure::
> +
> + struct kvm_memory_attributes {
> + __u64 address;
> + __u64 size;
> + __u64 attributes;
> + __u64 flags;
> + };
> +
> +The user sets the per-page memory attributes to a guest memory range indicated
> +by address/size, and in return KVM adjusts address and size to reflect the
> +actual pages of the memory range have been successfully set to the attributes.
> +If the call returns 0, "address" is updated to the last successful address + 1
> +and "size" is updated to the remaining address size that has not been set
> +successfully. The user should check the return value as well as the size to
> +decide if the operation succeeded for the whole range or not. The user may want
> +to retry the operation with the returned address/size if the previous range was
> +partially successful.
> +
> +Both address and size should be page aligned and the supported attributes can be
> +retrieved with KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES.
> +
> +The "flags" field may be used for future extensions and should be set to 0s.
> +
> 5. The kvm_run structure
> ========================
>
> @@ -8494,6 +8544,16 @@ block sizes is exposed in KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES as a
> 64-bit bitmap (each bit describing a block size). The default value is
> 0, to disable the eager page splitting.
>
> +8.41 KVM_CAP_MEMORY_ATTRIBUTES
> +------------------------------
> +
> +:Capability: KVM_CAP_MEMORY_ATTRIBUTES
> +:Architectures: x86
> +:Type: vm
> +
> +This capability indicates KVM supports per-page memory attributes and ioctls
> +KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES/KVM_SET_MEMORY_ATTRIBUTES are available.
> +
> 9. Known KVM API problems
> =========================
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index e9ca49d451f3..97db63da6227 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -264,6 +264,7 @@ struct kvm_gfn_range {
> gfn_t end;
> union {
> pte_t pte;
> + unsigned long attributes;
> u64 raw;
> } arg;
> bool may_block;
> @@ -809,6 +810,9 @@ struct kvm {
>
> #ifdef CONFIG_HAVE_KVM_PM_NOTIFIER
> struct notifier_block pm_notifier;
> +#endif
> +#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> + struct xarray mem_attr_array;
> #endif
> char stats_id[KVM_STATS_NAME_SIZE];
> };
> @@ -2301,4 +2305,14 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
> /* Max number of entries allowed for each kvm dirty ring */
> #define KVM_DIRTY_RING_MAX_ENTRIES 65536
>
> +#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> +static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
> +{
> + return xa_to_value(xa_load(&kvm->mem_attr_array, gfn));
> +}
> +
> +bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
> + struct kvm_gfn_range *range);
> +#endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */
> +
> #endif
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 6c6ed214b6ac..f065c57db327 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1211,6 +1211,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE 228
> #define KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES 229
> #define KVM_CAP_USER_MEMORY2 230
> +#define KVM_CAP_MEMORY_ATTRIBUTES 231
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> @@ -2270,4 +2271,17 @@ struct kvm_s390_zpci_op {
> /* flags for kvm_s390_zpci_op->u.reg_aen.flags */
> #define KVM_S390_ZPCIOP_REGAEN_HOST (1 << 0)
>
> +/* Available with KVM_CAP_MEMORY_ATTRIBUTES */
> +#define KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES _IOR(KVMIO, 0xd2, __u64)
> +#define KVM_SET_MEMORY_ATTRIBUTES _IOW(KVMIO, 0xd3, struct kvm_memory_attributes)
> +
> +struct kvm_memory_attributes {
> + __u64 address;
> + __u64 size;
> + __u64 attributes;
> + __u64 flags;
> +};
> +
> +#define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)
> +
> #endif /* __LINUX_KVM_H */
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 2fa11bd26cfc..8375bc49f97d 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -99,3 +99,7 @@ config KVM_GENERIC_HARDWARE_ENABLING
> config KVM_GENERIC_MMU_NOTIFIER
> select MMU_NOTIFIER
> bool
> +
> +config KVM_GENERIC_MEMORY_ATTRIBUTES
> + select KVM_GENERIC_MMU_NOTIFIER
> + bool
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c14adf93daec..1a31bfa025b0 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -530,6 +530,7 @@ struct kvm_mmu_notifier_range {
> u64 end;
> union {
> pte_t pte;
> + unsigned long attributes;
> u64 raw;
> } arg;
> gfn_handler_t handler;
> @@ -1175,6 +1176,9 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
> spin_lock_init(&kvm->mn_invalidate_lock);
> rcuwait_init(&kvm->mn_memslots_update_rcuwait);
> xa_init(&kvm->vcpu_array);
> +#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> + xa_init(&kvm->mem_attr_array);
> +#endif
>
> INIT_LIST_HEAD(&kvm->gpc_list);
> spin_lock_init(&kvm->gpc_lock);
> @@ -1346,6 +1350,9 @@ static void kvm_destroy_vm(struct kvm *kvm)
> kvm_free_memslots(kvm, &kvm->__memslots[i][0]);
> kvm_free_memslots(kvm, &kvm->__memslots[i][1]);
> }
> +#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> + xa_destroy(&kvm->mem_attr_array);
> +#endif
> cleanup_srcu_struct(&kvm->irq_srcu);
> cleanup_srcu_struct(&kvm->srcu);
> kvm_arch_free_vm(kvm);
> @@ -2346,6 +2353,145 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
> }
> #endif /* CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT */
>
> +#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> +static u64 kvm_supported_mem_attributes(struct kvm *kvm)
> +{
> + return 0;
> +}
> +
> +static __always_inline void kvm_handle_gfn_range(struct kvm *kvm,
> + struct kvm_mmu_notifier_range *range)
> +{
> + struct kvm_gfn_range gfn_range;
> + struct kvm_memory_slot *slot;
> + struct kvm_memslots *slots;
> + struct kvm_memslot_iter iter;
> + bool locked = false;
> + bool ret = false;
> + int i;
> +
> + gfn_range.arg.raw = range->arg.raw;
> + gfn_range.may_block = range->may_block;
> +
> + for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> + slots = __kvm_memslots(kvm, i);
> +
> + kvm_for_each_memslot_in_gfn_range(&iter, slots, range->start, range->end) {
> + slot = iter.slot;
> + gfn_range.slot = slot;
> +
> + gfn_range.start = max(range->start, slot->base_gfn);
> + gfn_range.end = min(range->end, slot->base_gfn + slot->npages);
> + if (gfn_range.start >= gfn_range.end)
> + continue;
> +
> + if (!locked) {
> + locked = true;
> + KVM_MMU_LOCK(kvm);
> + if (!IS_KVM_NULL_FN(range->on_lock))
> + range->on_lock(kvm);
> + }
> +
> + ret |= range->handler(kvm, &gfn_range);
> + }
> + }
> +
> + if (range->flush_on_ret && ret)
> + kvm_flush_remote_tlbs(kvm);
> +
> + if (locked) {
> + KVM_MMU_UNLOCK(kvm);
> + if (!IS_KVM_NULL_FN(range->on_unlock))
> + range->on_unlock(kvm);
> + }
> +}
> +
> +static int kvm_vm_set_mem_attributes(struct kvm *kvm, unsigned long attributes,
> + gfn_t start, gfn_t end)
> +{
> + struct kvm_mmu_notifier_range unmap_range = {
> + .start = start,
> + .end = end,
> + .handler = kvm_mmu_unmap_gfn_range,
> + .on_lock = kvm_mmu_invalidate_begin,
> + .on_unlock = (void *)kvm_null_fn,
> + .flush_on_ret = true,
> + .may_block = true,
> + };
> + struct kvm_mmu_notifier_range post_set_range = {
> + .start = start,
> + .end = end,
> + .arg.attributes = attributes,
> + .handler = kvm_arch_post_set_memory_attributes,
> + .on_lock = (void *)kvm_null_fn,
> + .on_unlock = kvm_mmu_invalidate_end,

on_unlock is called after unlocking mmu_lock. So kvm::mmu_invalidate_in_progress
is touched out side of it. Here is a quick fix.

WARNING: CPU: 108 PID: 62218 at arch/x86/kvm/../../../virt/kvm/kvm_main.c:757 kvm_mmu_unmap_gfn_range+0x32/0x70 [kvm]
...
RIP: 0010:kvm_mmu_unmap_gfn_range+0x32/0x70 [kvm]
...
Call Trace:
<TASK>
kvm_gmem_invalidate_begin+0xd0/0x130 [kvm]
kvm_gmem_fallocate+0x134/0x290 [kvm]
vfs_fallocate+0x151/0x380
__x64_sys_fallocate+0x3c/0x70
do_syscall_64+0x40/0x90
entry_SYSCALL_64_after_hwframe+0x6e/0xd8

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