KVM: Harden guest memory APIs against out-of-bounds accesses

When reading or writing a guest page, WARN and bail if offset+len would
result in a read to a different page so that KVM bugs are more likely to
be detected, and so that any such bugs are less likely to escalate to an
out-of-bounds access.  E.g. if userspace isn't using guard pages and the
target page is at the end of a memslot.

Note, KVM already hardens itself in similar APIs, e.g. in the "cached"
variants, it's just the vanilla APIs that are playing with fire.

Link: https://lore.kernel.org/r/20240829191413.900740-3-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

virt/kvm/kvm_main.c

@@ -3275,6 +3275,9 @@ static int __kvm_read_guest_page(struct kvm_memory_slot *slot, gfn_t gfn,
 	int r;
 	unsigned long addr;
 
+	if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+		return -EFAULT;
+
 	addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
 	if (kvm_is_error_hva(addr))
 		return -EFAULT;
@@ -3348,6 +3351,9 @@ static int __kvm_read_guest_atomic(struct kvm_memory_slot *slot, gfn_t gfn,
 	int r;
 	unsigned long addr;
 
+	if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+		return -EFAULT;
+
 	addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
 	if (kvm_is_error_hva(addr))
 		return -EFAULT;
@@ -3378,6 +3384,9 @@ static int __kvm_write_guest_page(struct kvm *kvm,
 	int r;
 	unsigned long addr;
 
+	if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+		return -EFAULT;
+
 	addr = gfn_to_hva_memslot(memslot, gfn);
 	if (kvm_is_error_hva(addr))
 		return -EFAULT;