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

[Chao Peng]

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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>
> > Tested-by: Fuad Tabba <tabba@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx/
[2]
https://lore.kernel.org/linux-mm/20221202061347.1070246-3-chao.p.peng@xxxxxxxxxxxxxxx/

Thanks,
Chao
> 
> [*] https://lore.kernel.org/linux-mm/20221025151344.3784230-3-chao.p.peng@xxxxxxxxxxxxxxx/
> 
> 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
> >