Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based private memory

Fuad Tabba
2022-12-06T12:40:13Z

Hi Chao,

On Tue, Dec 6, 2022 at 11:58 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
>
> On Mon, Dec 05, 2022 at 09:03:11AM +0000, Fuad Tabba wrote:
> > Hi Chao,
> >
> > On Fri, Dec 2, 2022 at 6:18 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> > >
> > > In memory encryption usage, guest memory may be encrypted with special
> > > key and can be accessed only by the guest itself. We call such memory
> > > private memory. It's valueless and sometimes can cause problem to allow
> > > userspace to access guest private memory. This new KVM memslot extension
> > > allows guest private memory being provided through a restrictedmem
> > > backed file descriptor(fd) and userspace is restricted to access the
> > > bookmarked memory in the fd.
> > >
> > > This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
> > > additional KVM memslot fields restricted_fd/restricted_offset to allow
> > > userspace to instruct KVM to provide guest memory through restricted_fd.
> > > 'guest_phys_addr' is mapped at the restricted_offset of restricted_fd
> > > and the size is 'memory_size'.
> > >
> > > The extended memslot can still have the userspace_addr(hva). When use, a
> > > single memslot can maintain both private memory through restricted_fd
> > > and shared memory through userspace_addr. Whether the private or shared
> > > part is visible to guest is maintained by other KVM code.
> > >
> > > A restrictedmem_notifier field is also added to the memslot structure to
> > > allow the restricted_fd's backing store to notify KVM the memory change,
> > > KVM then can invalidate its page table entries or handle memory errors.
> > >
> > > Together with the change, a new config HAVE_KVM_RESTRICTED_MEM is added
> > > and right now it is selected on X86_64 only.
> > >
> > > To make future maintenance easy, internally use a binary compatible
> > > alias struct kvm_user_mem_region to handle both the normal and the
> > > '_ext' variants.
> > >
> > > Co-developed-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> > > Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> > > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > > Reviewed-by: Fuad Tabba <tabba@google.com>
> > > Tested-by: Fuad Tabba <tabba@google.com>
> >
> > V9 of this patch [*] had KVM_CAP_PRIVATE_MEM, but it's not in this
> > patch series anymore. Any reason you removed it, or is it just an
> > omission?
>
> We had some discussion in v9 [1] to add generic memory attributes ioctls
> and KVM_CAP_PRIVATE_MEM can be implemented as a new
> KVM_MEMORY_ATTRIBUTE_PRIVATE flag via KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES()
> ioctl [2]. The api doc has been updated:
>
> +- KVM_MEM_PRIVATE, if KVM_MEMORY_ATTRIBUTE_PRIVATE is supported (see
> + KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES ioctl) …
>
>
> [1] https://lore.kernel.org/linux-mm/Y2WB48kD0J4VGynX@google.com/
> [2]
> https://lore.kernel.org/linux-mm/20221202061347.1070246-3-chao.p.peng@linux.intel.com/

I see. I just retested it with KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES,
and my Reviewed/Tested-by still apply.

Cheers,
/fuad

>
> Thanks,
> Chao
> >
> > [*] https://lore.kernel.org/linux-mm/20221025151344.3784230-3-chao.p.peng@linux.intel.com/
> >
> > Thanks,
> > /fuad
> >
> > > ---
> > > Documentation/virt/kvm/api.rst | 40 ++++++++++++++++++++++-----
> > > arch/x86/kvm/Kconfig | 2 ++
> > > arch/x86/kvm/x86.c | 2 +-
> > > include/linux/kvm_host.h | 8 ++++--
> > > include/uapi/linux/kvm.h | 28 +++++++++++++++++++
> > > virt/kvm/Kconfig | 3 +++
> > > virt/kvm/kvm_main.c | 49 ++++++++++++++++++++++++++++------
> > > 7 files changed, 114 insertions(+), 18 deletions(-)
> > >
> > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > > index bb2f709c0900..99352170c130 100644
> > > --- a/Documentation/virt/kvm/api.rst
> > > +++ b/Documentation/virt/kvm/api.rst
> > > @@ -1319,7 +1319,7 @@ yet and must be cleared on entry.
> > > :Capability: KVM_CAP_USER_MEMORY
> > > :Architectures: all
> > > :Type: vm ioctl
> > > -:Parameters: struct kvm_userspace_memory_region (in)
> > > +:Parameters: struct kvm_userspace_memory_region(_ext) (in)
> > > :Returns: 0 on success, -1 on error
> > >
> > > ::
> > > @@ -1332,9 +1332,18 @@ yet and must be cleared on entry.
> > > __u64 userspace_addr; /* start of the userspace allocated memory */
> > > };
> > >
> > > + struct kvm_userspace_memory_region_ext {
> > > + struct kvm_userspace_memory_region region;
> > > + __u64 restricted_offset;
> > > + __u32 restricted_fd;
> > > + __u32 pad1;
> > > + __u64 pad2[14];
> > > + };
> > > +
> > > /* for kvm_memory_region::flags */
> > > #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
> > > #define KVM_MEM_READONLY (1UL << 1)
> > > + #define KVM_MEM_PRIVATE (1UL << 2)
> > >
> > > This ioctl allows the user to create, modify or delete a guest physical
> > > memory slot. Bits 0-15 of "slot" specify the slot id and this value
> > > @@ -1365,12 +1374,29 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
> > > be identical. This allows large pages in the guest to be backed by large
> > > pages in the host.
> > >
> > > -The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and
> > > -KVM_MEM_READONLY. The former can be set to instruct KVM to keep track of
> > > -writes to memory within the slot. See KVM_GET_DIRTY_LOG ioctl to know how to
> > > -use it. The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,
> > > -to make a new slot read-only. In this case, writes to this memory will be
> > > -posted to userspace as KVM_EXIT_MMIO exits.
> > > +kvm_userspace_memory_region_ext struct includes all fields of
> > > +kvm_userspace_memory_region struct, while also adds additional fields for some
> > > +other features. See below description of flags field for more information.
> > > +It's recommended to use kvm_userspace_memory_region_ext in new userspace code.
> > > +
> > > +The flags field supports following flags:
> > > +
> > > +- KVM_MEM_LOG_DIRTY_PAGES to instruct KVM to keep track of writes to memory
> > > + within the slot. For more details, see KVM_GET_DIRTY_LOG ioctl.
> > > +
> > > +- KVM_MEM_READONLY, if KVM_CAP_READONLY_MEM allows, to make a new slot
> > > + read-only. In this case, writes to this memory will be posted to userspace as
> > > + KVM_EXIT_MMIO exits.
> > > +
> > > +- KVM_MEM_PRIVATE, if KVM_MEMORY_ATTRIBUTE_PRIVATE is supported (see
> > > + KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES ioctl), to indicate a new slot has private
> > > + memory backed by a file descriptor(fd) and userspace access to the fd may be
> > > + restricted. Userspace should use restricted_fd/restricted_offset in the
> > > + kvm_userspace_memory_region_ext to instruct KVM to provide private memory
> > > + to guest. Userspace should guarantee not to map the same host physical address
> > > + indicated by restricted_fd/restricted_offset to different guest physical
> > > + addresses within multiple memslots. Failed to do this may result undefined
> > > + behavior.
> > >
> > > When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of
> > > the memory region are automatically reflected into the guest. For example, an
> > > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> > > index a8e379a3afee..690cb21010e7 100644
> > > --- a/arch/x86/kvm/Kconfig
> > > +++ b/arch/x86/kvm/Kconfig
> > > @@ -50,6 +50,8 @@ config KVM
> > > select INTERVAL_TREE
> > > select HAVE_KVM_PM_NOTIFIER if PM
> > > select HAVE_KVM_MEMORY_ATTRIBUTES
> > > + select HAVE_KVM_RESTRICTED_MEM if X86_64
> > > + select RESTRICTEDMEM if HAVE_KVM_RESTRICTED_MEM
> > > help
> > > Support hosting fully virtualized guest machines using hardware
> > > virtualization extensions. You will need a fairly recent
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 7f850dfb4086..9a07380f8d3c 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -12224,7 +12224,7 @@ void __user * __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa,
> > > }
> > >
> > > for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> > > - struct kvm_userspace_memory_region m;
> > > + struct kvm_user_mem_region m;
> > >
> > > m.slot = id | (i << 16);
> > > m.flags = 0;
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index a784e2b06625..02347e386ea2 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -44,6 +44,7 @@
> > >
> > > #include <asm/kvm_host.h>
> > > #include <linux/kvm_dirty_ring.h>
> > > +#include <linux/restrictedmem.h>
> > >
> > > #ifndef KVM_MAX_VCPU_IDS
> > > #define KVM_MAX_VCPU_IDS KVM_MAX_VCPUS
> > > @@ -585,6 +586,9 @@ struct kvm_memory_slot {
> > > u32 flags;
> > > short id;
> > > u16 as_id;
> > > + struct file *restricted_file;
> > > + loff_t restricted_offset;
> > > + struct restrictedmem_notifier notifier;
> > > };
> > >
> > > static inline bool kvm_slot_dirty_track_enabled(const struct kvm_memory_slot *slot)
> > > @@ -1123,9 +1127,9 @@ enum kvm_mr_change {
> > > };
> > >
> > > int kvm_set_memory_region(struct kvm *kvm,
> > > - const struct kvm_userspace_memory_region *mem);
> > > + const struct kvm_user_mem_region *mem);
> > > int __kvm_set_memory_region(struct kvm *kvm,
> > > - const struct kvm_userspace_memory_region *mem);
> > > + const struct kvm_user_mem_region *mem);
> > > void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot);
> > > void kvm_arch_memslots_updated(struct kvm *kvm, u64 gen);
> > > int kvm_arch_prepare_memory_region(struct kvm *kvm,
> > > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > > index 5d0941acb5bb..13bff963b8b0 100644
> > > --- a/include/uapi/linux/kvm.h
> > > +++ b/include/uapi/linux/kvm.h
> > > @@ -103,6 +103,33 @@ struct kvm_userspace_memory_region {
> > > __u64 userspace_addr; /* start of the userspace allocated memory */
> > > };
> > >
> > > +struct kvm_userspace_memory_region_ext {
> > > + struct kvm_userspace_memory_region region;
> > > + __u64 restricted_offset;
> > > + __u32 restricted_fd;
> > > + __u32 pad1;
> > > + __u64 pad2[14];
> > > +};
> > > +
> > > +#ifdef __KERNEL__
> > > +/*
> > > + * kvm_user_mem_region is a kernel-only alias of kvm_userspace_memory_region_ext
> > > + * that "unpacks" kvm_userspace_memory_region so that KVM can directly access
> > > + * all fields from the top-level "extended" region.
> > > + */
> > > +struct kvm_user_mem_region {
> > > + __u32 slot;
> > > + __u32 flags;
> > > + __u64 guest_phys_addr;
> > > + __u64 memory_size;
> > > + __u64 userspace_addr;
> > > + __u64 restricted_offset;
> > > + __u32 restricted_fd;
> > > + __u32 pad1;
> > > + __u64 pad2[14];
> > > +};
> > > +#endif
> > > +
> > > /*
> > > * The bit 0 ~ bit 15 of kvm_memory_region::flags are visible for userspace,
> > > * other bits are reserved for kvm internal use which are defined in
> > > @@ -110,6 +137,7 @@ struct kvm_userspace_memory_region {
> > > */
> > > #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
> > > #define KVM_MEM_READONLY (1UL << 1)
> > > +#define KVM_MEM_PRIVATE (1UL << 2)
> > >
> > > /* for KVM_IRQ_LINE */
> > > struct kvm_irq_level {
> > > diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> > > index effdea5dd4f0..d605545d6dd1 100644
> > > --- a/virt/kvm/Kconfig
> > > +++ b/virt/kvm/Kconfig
> > > @@ -89,3 +89,6 @@ config KVM_XFER_TO_GUEST_WORK
> > >
> > > config HAVE_KVM_PM_NOTIFIER
> > > bool
> > > +
> > > +config HAVE_KVM_RESTRICTED_MEM
> > > + bool
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index 7f0f5e9f2406..b882eb2c76a2 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -1532,7 +1532,7 @@ static void kvm_replace_memslot(struct kvm *kvm,
> > > }
> > > }
> > >
> > > -static int check_memory_region_flags(const struct kvm_userspace_memory_region *mem)
> > > +static int check_memory_region_flags(const struct kvm_user_mem_region *mem)
> > > {
> > > u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
> > >
> > > @@ -1934,7 +1934,7 @@ static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
> > > * Must be called holding kvm->slots_lock for write.
> > > */
> > > int __kvm_set_memory_region(struct kvm *kvm,
> > > - const struct kvm_userspace_memory_region *mem)
> > > + const struct kvm_user_mem_region *mem)
> > > {
> > > struct kvm_memory_slot *old, *new;
> > > struct kvm_memslots *slots;
> > > @@ -2038,7 +2038,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > > EXPORT_SYMBOL_GPL(__kvm_set_memory_region);
> > >
> > > int kvm_set_memory_region(struct kvm *kvm,
> > > - const struct kvm_userspace_memory_region *mem)
> > > + const struct kvm_user_mem_region *mem)
> > > {
> > > int r;
> > >
> > > @@ -2050,7 +2050,7 @@ int kvm_set_memory_region(struct kvm *kvm,
> > > EXPORT_SYMBOL_GPL(kvm_set_memory_region);
> > >
> > > static int kvm_vm_ioctl_set_memory_region(struct kvm *kvm,
> > > - struct kvm_userspace_memory_region *mem)
> > > + struct kvm_user_mem_region *mem)
> > > {
> > > if ((u16)mem->slot >= KVM_USER_MEM_SLOTS)
> > > return -EINVAL;
> > > @@ -4698,6 +4698,33 @@ static int kvm_vm_ioctl_get_stats_fd(struct kvm *kvm)
> > > return fd;
> > > }
> > >
> > > +#define SANITY_CHECK_MEM_REGION_FIELD(field) \
> > > +do { \
> > > + BUILD_BUG_ON(offsetof(struct kvm_user_mem_region, field) != \
> > > + offsetof(struct kvm_userspace_memory_region, field)); \
> > > + BUILD_BUG_ON(sizeof_field(struct kvm_user_mem_region, field) != \
> > > + sizeof_field(struct kvm_userspace_memory_region, field)); \
> > > +} while (0)
> > > +
> > > +#define SANITY_CHECK_MEM_REGION_EXT_FIELD(field) \
> > > +do { \
> > > + BUILD_BUG_ON(offsetof(struct kvm_user_mem_region, field) != \
> > > + offsetof(struct kvm_userspace_memory_region_ext, field)); \
> > > + BUILD_BUG_ON(sizeof_field(struct kvm_user_mem_region, field) != \
> > > + sizeof_field(struct kvm_userspace_memory_region_ext, field)); \
> > > +} while (0)
> > > +
> > > +static void kvm_sanity_check_user_mem_region_alias(void)
> > > +{
> > > + SANITY_CHECK_MEM_REGION_FIELD(slot);
> > > + SANITY_CHECK_MEM_REGION_FIELD(flags);
> > > + SANITY_CHECK_MEM_REGION_FIELD(guest_phys_addr);
> > > + SANITY_CHECK_MEM_REGION_FIELD(memory_size);
> > > + SANITY_CHECK_MEM_REGION_FIELD(userspace_addr);
> > > + SANITY_CHECK_MEM_REGION_EXT_FIELD(restricted_offset);
> > > + SANITY_CHECK_MEM_REGION_EXT_FIELD(restricted_fd);
> > > +}
> > > +
> > > static long kvm_vm_ioctl(struct file *filp,
> > > unsigned int ioctl, unsigned long arg)
> > > {
> > > @@ -4721,14 +4748,20 @@ static long kvm_vm_ioctl(struct file *filp,
> > > break;
> > > }
> > > case KVM_SET_USER_MEMORY_REGION: {
> > > - struct kvm_userspace_memory_region kvm_userspace_mem;
> > > + struct kvm_user_mem_region mem;
> > > + unsigned long size = sizeof(struct kvm_userspace_memory_region);
> > > +
> > > + kvm_sanity_check_user_mem_region_alias();
> > >
> > > r = -EFAULT;
> > > - if (copy_from_user(&kvm_userspace_mem, argp,
> > > - sizeof(kvm_userspace_mem)))
> > > + if (copy_from_user(&mem, argp, size))
> > > + goto out;
> > > +
> > > + r = -EINVAL;
> > > + if (mem.flags & KVM_MEM_PRIVATE)
> > > goto out;
> > >
> > > - r = kvm_vm_ioctl_set_memory_region(kvm, &kvm_userspace_mem);
> > > + r = kvm_vm_ioctl_set_memory_region(kvm, &mem);
> > > break;
> > > }
> > > case KVM_GET_DIRTY_LOG: {
> > > --
> > > 2.25.1
> > >