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>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 66548287ed424824513766b78c1833482f7fd626..e38705359af5e436a2fa29b5c2c9ff0f28226103 100644 (file)
--- 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;
 

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 1c68384a7c4b8eec5dc9b18e1c4efff84c438bac..538fd57ea3393b486a037333d8a357d92e7c86c1 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1638,6 +1638,15 @@ 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!!! */
@@ -1668,6 +1677,9 @@ 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;
        /*
@@ -1751,6 +1763,13 @@ 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) ||