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Commit 75d61fbc authored by Takuya Yoshikawa's avatar Takuya Yoshikawa Committed by Marcelo Tosatti
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KVM: set_memory_region: Disallow changing read-only attribute later 



As Xiao pointed out, there are a few problems with it:
 - kvm_arch_commit_memory_region() write protects the memory slot only
   for GET_DIRTY_LOG when modifying the flags.
 - FNAME(sync_page) uses the old spte value to set a new one without
   checking KVM_MEM_READONLY flag.

Since we flush all shadow pages when creating a new slot, the simplest
fix is to disallow such problematic flag changes: this is safe because
no one is doing such things.

Reviewed-by: default avatarGleb Natapov <gleb@redhat.com>
Signed-off-by: default avatarTakuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent f64c0398

Documentation/virtual/kvm/api.txt

+6 −6
Original line number Diff line number Diff line
@@ -874,12 +874,12 @@ 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 two flag, KVM_MEM_LOG_DIRTY_PAGES, which instructs

kvm to keep track of writes to memory within the slot.  See KVM_GET_DIRTY_LOG

ioctl.  The KVM_CAP_READONLY_MEM capability indicates the availability of the

KVM_MEM_READONLY flag.  When this flag is set for a memory region, KVM only

allows read accesses.  Writes will be posted to userspace as KVM_EXIT_MMIO

exits.

The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and

KVM_MEM_READONLY.  The former can be set to instruct KVM to keep track of

writes to memory within the slot.  See KVM_GET_DIRTY_LOG ioctl to know how to

use it.  The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,

to make a new slot read-only.  In this case, writes to this memory will be

posted to userspace as KVM_EXIT_MMIO exits.



When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of

the memory region are automatically reflected into the guest.  For example, an

virt/kvm/kvm_main.c

+12 −23
Original line number Diff line number Diff line
@@ -754,7 +754,6 @@ int __kvm_set_memory_region(struct kvm *kvm, 
	struct kvm_memory_slot *slot;

	struct kvm_memory_slot old, new;

	struct kvm_memslots *slots = NULL, *old_memslots;

	bool old_iommu_mapped;

	enum kvm_mr_change change;



	r = check_memory_region_flags(mem);
@@ -797,15 +796,14 @@ int __kvm_set_memory_region(struct kvm *kvm, 
	new.npages = npages;

	new.flags = mem->flags;



	old_iommu_mapped = old.npages;



	r = -EINVAL;

	if (npages) {

		if (!old.npages)

			change = KVM_MR_CREATE;

		else { /* Modify an existing slot. */

			if ((mem->userspace_addr != old.userspace_addr) ||

			    (npages != old.npages))

			    (npages != old.npages) ||

			    ((new.flags ^ old.flags) & KVM_MEM_READONLY))

				goto out;



			if (base_gfn != old.base_gfn)
@@ -867,7 +865,6 @@ int __kvm_set_memory_region(struct kvm *kvm, 


		/* slot was deleted or moved, clear iommu mapping */

		kvm_iommu_unmap_pages(kvm, &old);

		old_iommu_mapped = false;

		/* From this point no new shadow pages pointing to a deleted,

		 * or moved, memslot will be created.

		 *
@@ -898,26 +895,18 @@ int __kvm_set_memory_region(struct kvm *kvm, 


	/*

	 * IOMMU mapping:  New slots need to be mapped.  Old slots need to be

	 * un-mapped and re-mapped if their base changes or if flags that the

	 * iommu cares about change (read-only).  Base change unmapping is

	 * handled above with slot deletion, so we only unmap incompatible

	 * flags here.  Anything else the iommu might care about for existing

	 * slots (size changes, userspace addr changes) is disallowed above,

	 * so any other attribute changes getting here can be skipped.

	 * un-mapped and re-mapped if their base changes.  Since base change

	 * unmapping is handled above with slot deletion, mapping alone is

	 * needed here.  Anything else the iommu might care about for existing

	 * slots (size changes, userspace addr changes and read-only flag

	 * changes) is disallowed above, so any other attribute changes getting

	 * here can be skipped.

	 */

	if (change != KVM_MR_DELETE) {

		if (old_iommu_mapped &&

		    ((new.flags ^ old.flags) & KVM_MEM_READONLY)) {

			kvm_iommu_unmap_pages(kvm, &old);

			old_iommu_mapped = false;

		}



		if (!old_iommu_mapped) {

	if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE)) {

		r = kvm_iommu_map_pages(kvm, &new);

		if (r)

			goto out_slots;

	}

	}



	/* actual memory is freed via old in kvm_free_physmem_slot below */

	if (change == KVM_MR_DELETE) {