Re: [PATCH v7 09/14] KVM: Extend the memslot to support fd-based private memory

[Chao Peng]

On Fri, Jul 29, 2022 at 07:51:29PM +0000, Sean Christopherson wrote:
> On Wed, Jul 06, 2022, Chao Peng wrote:
> > @@ -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 private_offset;
> > +   __u32 private_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)
> 
> Very belatedly following up on prior feedback...
> 
>   | I think a flag is still needed, the problem is private_fd can be safely
>   | accessed only when this flag is set, e.g. without this flag, we can't
>   | copy_from_user these new fields since they don't exist for previous
>   | kvm_userspace_memory_region callers.
> 
> I forgot about that aspect of things.  We don't technically need a dedicated
> PRIVATE flag to handle that, but it does seem to be the least awful soltuion.
> We could either add a generic KVM_MEM_EXTENDED_REGION or an entirely new
> ioctl(), e.g. KVM_SET_USER_MEMORY_REGION2, but in both approaches there's a 
> decent
> chance that we'll end up needed individual "this field is valid" flags anways.
> 
> E.g. if KVM requires pad1 and pad2 to be zero to carve out future extensions,
> then we're right back here if some future extension needs to treat '0' as a 
> legal
> input.

I had such practice (always rejecting none-zero 'pad' value when
introducing new user APIs) in other project previously, but I rarely
see that in KVM.

> 
> TL;DR: adding KVM_MEM_PRIVATE still seems like the best approach.
> 
> > @@ -4631,14 +4658,35 @@ 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;
> > +           u32 flags;
> > +
> > +           kvm_sanity_check_user_mem_region_alias();
> > +
> > +           memset(&mem, 0, sizeof(mem));
> >  
> >             r = -EFAULT;
> > -           if (copy_from_user(&kvm_userspace_mem, argp,
> > -                                           sizeof(kvm_userspace_mem)))
> > +
> > +           if (get_user(flags,
> > +                   (u32 __user *)(argp + offsetof(typeof(mem), flags))))
> > +                   goto out;
> 
> 
> Indentation is funky.  It's hard to massage this into something short and
> readable  What about capturing the offset separately?  E.g.
> 
>                 struct kvm_user_mem_region mem;
>                 unsigned int flags_offset = offsetof(typeof(mem), flags));
>                 unsigned long size;
>                 u32 flags;
> 
>                 kvm_sanity_check_user_mem_region_alias();
> 
>               memset(&mem, 0, sizeof(mem));
> 
>                 r = -EFAULT;
>                 if (get_user(flags, (u32 __user *)(argp + flags_offset)))
>                         goto out;
> 
> But this can actually be punted until KVM_MEM_PRIVATE is fully supported.  As 
> of
> this patch, KVM doesn't read the extended size, so I believe the diff for this
> patch can simply be:

Looks good to me, Thanks.

Chao
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index da263c370d00..5194beb7b52f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4640,6 +4640,10 @@ static long kvm_vm_ioctl(struct file *filp,
>                                                 sizeof(kvm_userspace_mem)))
>                         goto out;
> 
> +               r = -EINVAL;
> +               if (mem.flags & KVM_MEM_PRIVATE)
> +                       goto out;
> +
>                 r = kvm_vm_ioctl_set_memory_region(kvm, &kvm_userspace_mem);
>                 break;
>         }