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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: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Takuya 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: Marcelo Tosatti <mtosatti@redhat.com>
This commit is contained in:
Takuya Yoshikawa 2013-01-30 19:40:41 +09:00 committed by Marcelo Tosatti
parent f64c039893
commit 75d61fbcf5
2 changed files with 18 additions and 29 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
Documentation/virtual/kvm/api.txt
View File

@ -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

35
virt/kvm/kvm_main.c
View File

@ -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.
*
@ -898,25 +895,17 @@ 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) {
r = kvm_iommu_map_pages(kvm, &new);
if (r)
goto out_slots;
}
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 */