Re: [PATCH v6 4/8] KVM: Extend the memslot to support fd-based private memory

[Sean Christopherson]

On Thu, May 19, 2022, Chao Peng wrote:
> @@ -653,12 +662,12 @@ struct kvm_irq_routing_table {
>  };
>  #endif
>  
> -#ifndef KVM_PRIVATE_MEM_SLOTS
> -#define KVM_PRIVATE_MEM_SLOTS 0
> +#ifndef KVM_INTERNAL_MEM_SLOTS
> +#define KVM_INTERNAL_MEM_SLOTS 0
>  #endif

This rename belongs in a separate patch.

>  #define KVM_MEM_SLOTS_NUM SHRT_MAX
> -#define KVM_USER_MEM_SLOTS (KVM_MEM_SLOTS_NUM - KVM_PRIVATE_MEM_SLOTS)
> +#define KVM_USER_MEM_SLOTS (KVM_MEM_SLOTS_NUM - KVM_INTERNAL_MEM_SLOTS)
>  
>  #ifndef __KVM_VCPU_MULTIPLE_ADDRESS_SPACE
>  static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu)
> @@ -1087,9 +1096,9 @@ 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 e10d131edd80..28cacd3656d4 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -103,6 +103,29 @@ 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 private_offset;
> +     __u32 private_fd;
> +     __u32 pad1;
> +     __u64 pad2[14];
> +};
> +
> +#ifdef __KERNEL__
> +/* Internal helper, the layout must match above user visible structures */

It's worth explicity calling out which structureso this aliases.  And rather 
than
add a comment about the layout needing to match that, enforce it in code. I
personally wouldn't bother with an expolicit comment about the layout, IMO 
that's
a fairly obvious implication of aliasing.

/*
 * 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.
 */


And I think it's in this patch that you missed a conversion to the alias, in the
prototype for check_memory_region_flags() (looks like it gets fixed up later in
the series).

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0f81bf0407be..8765b334477d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1466,7 +1466,7 @@ 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;

@@ -4514,6 +4514,33 @@ 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(private_offset);
+       SANITY_CHECK_MEM_REGION_EXT_FIELD(private_fd);
+}
+
 static long kvm_vm_ioctl(struct file *filp,
                           unsigned int ioctl, unsigned long arg)
 {
@@ -4541,6 +4568,8 @@ static long kvm_vm_ioctl(struct file *filp,
                unsigned long size;
                u32 flags;

+               kvm_sanity_check_user_mem_region_alias();
+
                memset(&mem, 0, sizeof(mem));

                r = -EFAULT;

> +struct kvm_user_mem_region {
> +     __u32 slot;
> +     __u32 flags;
> +     __u64 guest_phys_addr;
> +     __u64 memory_size;
> +     __u64 userspace_addr;
> +     __u64 private_offset;
> +     __u32 private_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 +133,7 @@ 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)

Hmm, KVM_MEM_PRIVATE is technically wrong now that a "private" memslot maps 
private
and/or shared memory.  Strictly speaking, we don't actually need a new flag.  
Valid
file descriptors must be >=0, so the logic for specifying a memslot that can be
converted between private and shared could be that "(int)private_fd < 0" means
"not convertible", i.e. derive the flag from private_fd.

And looking at the two KVM consumers of the flag, via kvm_slot_is_private(), 
they're
both wrong.  Both kvm_faultin_pfn() and kvm_mmu_max_mapping_level() should 
operate
on the _fault_, not the slot.  So it would actually be a positive to not have 
an easy
way to query if a slot supports conversion.

>  /* for KVM_IRQ_LINE */
>  struct kvm_irq_level {

...

> +             if (flags & KVM_MEM_PRIVATE) {

An added bonus of dropping KVM_MEM_PRIVATE is that these checks go away.

> +                     r = -EINVAL;
> +                     goto out;
> +             }
> +
> +             size = sizeof(struct kvm_userspace_memory_region);
> +
> +             if (copy_from_user(&mem, argp, size))
> +                     goto out;
> +
> +             r = -EINVAL;
> +             if ((flags ^ 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
>