KVM: Require total number of memslot pages to fit in an unsigned long

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@google.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <1c2c91baf8e78acccd4dad38da591002e61c013c.1638817638.git.maciej.szmigiero@oracle.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 66548287ed42..e38705359af5 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -552,6 +552,7 @@ 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;