Re: [PATCH v2] KVM: introduce readonly memory region

[Gleb Natapov]

On Thu, May 24, 2012 at 05:24:34PM +0800, Xiao Guangrong wrote:
> In current code, if we map a readonly memory space from host to guest
> and the page is not currently mapped in the host, we will get a fault-pfn
> and async is not allowed, then the vm will crash
> 
> Address Avi's idea, we introduce readonly memory region to map ROM/ROMD
> to the guest
> 
As far as I can tell this implements only ROMD. i.e write access to read
only slot will generate IO exit.

> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
>  Documentation/virtual/kvm/api.txt |    9 +++++--
>  include/linux/kvm.h               |    5 ++-
>  virt/kvm/kvm_main.c               |   43 ++++++++++++++++++++++++++++++-------
>  3 files changed, 44 insertions(+), 13 deletions(-)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 9301266..e2a82c3 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -857,7 +857,8 @@ struct kvm_userspace_memory_region {
>  };
> 
>  /* for kvm_memory_region::flags */
> -#define KVM_MEM_LOG_DIRTY_PAGES  1UL
> +#define KVM_MEM_LOG_DIRTY_PAGES		1UL
> +#define KVM_MEM_READ_ONLY		(1UL << 2)
> 
>  This ioctl allows the user to create or modify a guest physical memory
>  slot.  When changing an existing slot, it may be moved in the guest
> @@ -873,9 +874,11 @@ 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 just one flag, KVM_MEM_LOG_DIRTY_PAGES, which
> +The flags field supports two flags, KVM_MEM_LOG_DIRTY_PAGES, which
>  instructs kvm to keep track of writes to memory within the slot.  See
> -the KVM_GET_DIRTY_LOG ioctl.
> +the KVM_GET_DIRTY_LOG ioctl. Another flag is KVM_MEM_READ_ONLY, which
> +indicates the guest memory is read-only, that means, guest is only allowed
> +to read it.
> 
>  When the KVM_CAP_SYNC_MMU capability, changes in the backing of the memory
>  region are automatically reflected into the guest.  For example, an mmap()
> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> index 09f2b3a..d178e3d 100644
> --- a/include/linux/kvm.h
> +++ b/include/linux/kvm.h
> @@ -102,8 +102,9 @@ struct kvm_userspace_memory_region {
>  };
> 
>  /* for kvm_memory_region::flags */
> -#define KVM_MEM_LOG_DIRTY_PAGES  1UL
> -#define KVM_MEMSLOT_INVALID      (1UL << 1)
> +#define KVM_MEM_LOG_DIRTY_PAGES		1UL
> +#define KVM_MEMSLOT_INVALID		(1UL << 1)
> +#define KVM_MEM_READ_ONLY		(1UL << 2)
> 
>  /* for KVM_IRQ_LINE */
>  struct kvm_irq_level {
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 7e14068..27283e4 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1009,10 +1009,11 @@ out:
>  	return size;
>  }
> 
> -static unsigned long gfn_to_hva_many(struct kvm_memory_slot *slot, gfn_t gfn,
> -				     gfn_t *nr_pages)
> +static unsigned long __gfn_to_hva_many(struct kvm_memory_slot *slot, gfn_t gfn,
> +				       gfn_t *nr_pages, bool write)
>  {
> -	if (!slot || slot->flags & KVM_MEMSLOT_INVALID)
> +	if (!slot || slot->flags & KVM_MEMSLOT_INVALID ||
> +	      ((slot->flags & KVM_MEM_READ_ONLY) && write))
>  		return bad_hva();
> 
>  	if (nr_pages)
> @@ -1021,6 +1022,17 @@ static unsigned long gfn_to_hva_many(struct kvm_memory_slot *slot, gfn_t gfn,
>  	return gfn_to_hva_memslot(slot, gfn);
>  }
> 
> +static unsigned long gfn_to_hva_many(struct kvm_memory_slot *slot, gfn_t gfn,
> +				     gfn_t *nr_pages)
> +{
> +	return __gfn_to_hva_many(slot, gfn, nr_pages, true);
> +}
> +
> +unsigned long gfn_to_hva_prot(struct kvm *kvm, gfn_t gfn, bool write)
> +{
> +	return __gfn_to_hva_many(gfn_to_memslot(kvm, gfn), gfn, NULL, write);
> +}
> +
>  unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn)
>  {
>  	return gfn_to_hva_many(gfn_to_memslot(kvm, gfn), gfn, NULL);
> @@ -1053,6 +1065,21 @@ static inline int check_user_page_hwpoison(unsigned long addr)
>  	return rc == -EHWPOISON;
>  }
> 
> +static bool vma_is_avalid(struct vm_area_struct *vma, bool write_fault)
> +{
> +	if (write_fault) {
> +		if (unlikely(!(vma->vm_flags & VM_WRITE)))
> +			return false;
> +
> +		return true;
> +	}
> +
> +	if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))))
> +		return false;
> +
> +	return true;
> +}
> +
>  static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
>  			bool *async, bool write_fault, bool *writable)
>  {
> @@ -1076,7 +1103,6 @@ static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
> 
>  		if (writable)
>  			*writable = write_fault;
> -
>  		if (async) {
>  			down_read(&current->mm->mmap_sem);
>  			npages = get_user_page_nowait(current, current->mm,
> @@ -1123,8 +1149,9 @@ static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
>  				vma->vm_pgoff;
>  			BUG_ON(!kvm_is_mmio_pfn(pfn));
>  		} else {
> -			if (async && (vma->vm_flags & VM_WRITE))
> +			if (async && vma_is_avalid(vma, write_fault))
>  				*async = true;
> +
>  			pfn = get_fault_pfn();
>  		}
>  		up_read(&current->mm->mmap_sem);
> @@ -1148,7 +1175,7 @@ static pfn_t __gfn_to_pfn(struct kvm *kvm, gfn_t gfn, bool atomic, bool *async,
>  	if (async)
>  		*async = false;
> 
> -	addr = gfn_to_hva(kvm, gfn);
> +	addr = gfn_to_hva_prot(kvm, gfn, write_fault);
>  	if (kvm_is_error_hva(addr)) {
>  		get_page(bad_page);
>  		return page_to_pfn(bad_page);
> @@ -1293,7 +1320,7 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
>  	int r;
>  	unsigned long addr;
> 
> -	addr = gfn_to_hva(kvm, gfn);
> +	addr = gfn_to_hva_prot(kvm, gfn, false);
>  	if (kvm_is_error_hva(addr))
>  		return -EFAULT;
>  	r = __copy_from_user(data, (void __user *)addr + offset, len);
> @@ -1331,7 +1358,7 @@ int kvm_read_guest_atomic(struct kvm *kvm, gpa_t gpa, void *data,
>  	gfn_t gfn = gpa >> PAGE_SHIFT;
>  	int offset = offset_in_page(gpa);
> 
> -	addr = gfn_to_hva(kvm, gfn);
> +	addr = gfn_to_hva_prot(kvm, gfn, false);
>  	if (kvm_is_error_hva(addr))
>  		return -EFAULT;
>  	pagefault_disable();
> -- 
> 1.7.7.6
> 
> --
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--
			Gleb.
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