Re: [PATCH] KVM: arm: remove excessive permission check in kvm_arch_prepare_memory_region

[Christoffer Dall]

On Fri, Dec 06, 2019 at 10:08:02AM +0800, Jia He wrote:
> In kvm_arch_prepare_memory_region, arm kvm regards the memory region as
> writable if the flag has no KVM_MEM_READONLY, and the vm is readonly if
> !VM_WRITE.
> 
> But there is common usage for setting kvm memory region as follows:
> e.g. qemu side (see the PROT_NONE flag)
> 1. mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>    memory_region_init_ram_ptr()
> 2. re mmap the above area with read/write authority.
> 
> Such example is used in virtio-fs qemu codes which hasn't been upstreamed
> [1]. But seems we can't forbid this example.
> 
> Without this patch, it will cause an EPERM during kvm_set_memory_region()
> and cause qemu boot crash.
> 
> As told by Ard, "the underlying assumption is incorrect, i.e., that the
> value of vm_flags at this point in time defines how the VMA is used
> during its lifetime. There may be other cases where a VMA is created
> with VM_READ vm_flags that are changed to VM_READ|VM_WRITE later, and
> we are currently rejecting this use case as well."
> 
> [1] https://gitlab.com/virtio-fs/qemu/blob/5a356e/hw/virtio/vhost-user-fs.c#L488

Reviewed-by: Christoffer Dall <christoffer.dall@xxxxxxx>

> 
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Suggested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Signed-off-by: Jia He <justin.he@xxxxxxx>
> ---
>  virt/kvm/arm/mmu.c | 9 ---------
>  1 file changed, 9 deletions(-)
> 
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 38b4c910b6c3..a48994af70b8 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -2301,15 +2301,6 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  		if (!vma || vma->vm_start >= reg_end)
>  			break;
>  
> -		/*
> -		 * Mapping a read-only VMA is only allowed if the
> -		 * memory region is configured as read-only.
> -		 */
> -		if (writable && !(vma->vm_flags & VM_WRITE)) {
> -			ret = -EPERM;
> -			break;
> -		}
> -
>  		/*
>  		 * Take the intersection of this VMA with the memory region
>  		 */
> -- 
> 2.17.1
> 
> 
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