KVM: mmu: Fix overlap between public and private memslots

Reported by syzkaller:

    pte_list_remove: ffff9714eb1f8078 0->BUG
    ------------[ cut here ]------------
    kernel BUG at arch/x86/kvm/mmu.c:1157!
    invalid opcode: 0000 [#1] SMP
    RIP: 0010:pte_list_remove+0x11b/0x120 [kvm]
    Call Trace:
     drop_spte+0x83/0xb0 [kvm]
     mmu_page_zap_pte+0xcc/0xe0 [kvm]
     kvm_mmu_prepare_zap_page+0x81/0x4a0 [kvm]
     kvm_mmu_invalidate_zap_all_pages+0x159/0x220 [kvm]
     kvm_arch_flush_shadow_all+0xe/0x10 [kvm]
     kvm_mmu_notifier_release+0x6c/0xa0 [kvm]
     ? kvm_mmu_notifier_release+0x5/0xa0 [kvm]
     __mmu_notifier_release+0x79/0x110
     ? __mmu_notifier_release+0x5/0x110
     exit_mmap+0x15a/0x170
     ? do_exit+0x281/0xcb0
     mmput+0x66/0x160
     do_exit+0x2c9/0xcb0
     ? __context_tracking_exit.part.5+0x4a/0x150
     do_group_exit+0x50/0xd0
     SyS_exit_group+0x14/0x20
     do_syscall_64+0x73/0x1f0
     entry_SYSCALL64_slow_path+0x25/0x25

The reason is that when creates new memslot, there is no guarantee for new
memslot not overlap with private memslots. This can be triggered by the
following program:

   #include <fcntl.h>
   #include <pthread.h>
   #include <setjmp.h>
   #include <signal.h>
   #include <stddef.h>
   #include <stdint.h>
   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>
   #include <sys/ioctl.h>
   #include <sys/stat.h>
   #include <sys/syscall.h>
   #include <sys/types.h>
   #include <unistd.h>
   #include <linux/kvm.h>

   long r[16];

   int main()
   {
	void *p = valloc(0x4000);

	r[2] = open("/dev/kvm", 0);
	r[3] = ioctl(r[2], KVM_CREATE_VM, 0x0ul);

	uint64_t addr = 0xf000;
	ioctl(r[3], KVM_SET_IDENTITY_MAP_ADDR, &addr);
	r[6] = ioctl(r[3], KVM_CREATE_VCPU, 0x0ul);
	ioctl(r[3], KVM_SET_TSS_ADDR, 0x0ul);
	ioctl(r[6], KVM_RUN, 0);
	ioctl(r[6], KVM_RUN, 0);

	struct kvm_userspace_memory_region mr = {
		.slot = 0,
		.flags = KVM_MEM_LOG_DIRTY_PAGES,
		.guest_phys_addr = 0xf000,
		.memory_size = 0x4000,
		.userspace_addr = (uintptr_t) p
	};
	ioctl(r[3], KVM_SET_USER_MEMORY_REGION, &mr);
	return 0;
   }

This patch fixes the bug by not adding a new memslot even if it
overlaps with private memslots.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 virt/kvm/kvm_main.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

virt/kvm/kvm_main.c

@@ -969,8 +969,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		/* Check for overlaps */
 		r = -EEXIST;
 		kvm_for_each_memslot(slot, __kvm_memslots(kvm, as_id)) {
-			if ((slot->id >= KVM_USER_MEM_SLOTS) ||
-			    (slot->id == id))
+			if (slot->id == id)
 				continue;
 			if (!((base_gfn + npages <= slot->base_gfn) ||
 			      (base_gfn >= slot->base_gfn + slot->npages)))