KVM: arm/arm64: Remove excessive permission check in kvm_arch_prepare_memory_region

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

Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Jia He <justin.he@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Link: https://lore.kernel.org/r/20191206020802.196108-1-justin.he@arm.com

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
 		 */