From: Jia He <justin.he@arm.com>
Date: Fri, 6 Dec 2019 02:08:02 +0000 (+0800)
Subject: KVM: arm/arm64: Remove excessive permission check in kvm_arch_prepare_memory_region
X-Git-Tag: v5.15~4838^2~3^2~2
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=97418e968b01ba8e3ad41c38b42106c48bc19544;p=platform%2Fkernel%2Flinux-starfive.git

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
---

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index 38b4c91..a48994a 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -2302,15 +2302,6 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 			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
 		 */
 		vm_start = max(hva, vma->vm_start);