From: Sean Christopherson <seanjc@xxxxxxxxxx>
Explicitly disallow creating more memslot pages than can fit in an
unsigned long, KVM doesn't correctly handle a total number of memslot
pages that doesn't fit in an unsigned long and remedying that would be a
waste of time.
For a 64-bit kernel, this is a nop as memslots are not allowed to overlap
in the gfn address space.
With a 32-bit kernel, userspace can at most address 3gb of virtual memory,
whereas wrapping the total number of pages would require 4tb+ of guest
physical memory. Even with x86's second address space for SMM, userspace
would need to alias all of guest memory more than one _thousand_ times.
And on older x86 hardware with MAXPHYADDR < 43, the guest couldn't
actually access any of those aliases even if userspace lied about
guest.MAXPHYADDR.
On 390 and arm64, this is a nop as they don't support 32-bit hosts.
On x86, practically speaking this is simply acknowledging reality as the
existing kvm_mmu_calculate_default_mmu_pages() assumes the total number
of pages fits in an "unsigned long".
On PPC, this is likely a nop as every flavor of PPC KVM assumes gfns (and
gpas!) fit in unsigned long. arch/powerpc/kvm/book3s_32_mmu_host.c goes
a step further and fails the build if CONFIG_PTE_64BIT=y, which
presumably means that it does't support 64-bit physical addresses.
On MIPS, this is also likely a nop as the core MMU helpers assume gpas
fit in unsigned long, e.g. see kvm_mips_##name##_pte.
And finally, RISC-V is a "don't care" as it doesn't exist in any release,
i.e. there is no established ABI to break.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
---
include/linux/kvm_host.h | 1 +
virt/kvm/kvm_main.c | 19 +++++++++++++++++++
2 files changed, 20 insertions(+)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index a745efe389ab..a6830966f8eb 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -554,6 +554,7 @@ struct kvm {
*/
struct mutex slots_arch_lock;
struct mm_struct *mm; /* userspace tied to this vm */
+ unsigned long nr_memslot_pages;
struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
struct xarray vcpu_array;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 863112783ed9..4a1b484518a9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1640,6 +1640,15 @@ static int kvm_set_memslot(struct kvm *kvm,
update_memslots(slots, new, change);
slots = install_new_memslots(kvm, as_id, slots);
+ /*
+ * Update the total number of memslot pages before calling the arch
+ * hook so that architectures can consume the result directly.
+ */
+ if (change == KVM_MR_DELETE)
+ kvm->nr_memslot_pages -= old.npages;
+ else if (change == KVM_MR_CREATE)
+ kvm->nr_memslot_pages += new->npages;
+
kvm_arch_commit_memory_region(kvm, mem, &old, new, change);
/* Free the old memslot's metadata. Note, this is the full copy!!! */
@@ -1670,6 +1679,9 @@ static int kvm_delete_memslot(struct kvm *kvm,
if (!old->npages)
return -EINVAL;
+ if (WARN_ON_ONCE(kvm->nr_memslot_pages < old->npages))
+ return -EIO;
+
memset(&new, 0, sizeof(new));
new.id = old->id;
/*
@@ -1753,6 +1765,13 @@ int __kvm_set_memory_region(struct kvm *kvm,
if (!old.npages) {
change = KVM_MR_CREATE;
new.dirty_bitmap = NULL;
+
+ /*
+ * To simplify KVM internals, the total number of pages across
+ * all memslots must fit in an unsigned long.
+ */
+ if ((kvm->nr_memslot_pages + new.npages) < kvm->nr_memslot_pages)
+ return -EINVAL;
} else { /* Modify an existing slot. */
if ((new.userspace_addr != old.userspace_addr) ||
(new.npages != old.npages) ||
