kvm: access to invalid memory in mmu_zap_unsync_children

[Sasha Levin]

Hi all,

While fuzzing with syzkaller on the latest -next kernel running on a KVM tools
guest, I've hit the following invalid memory access:

[  547.956284] UBSAN: Undefined behaviour in arch/x86/kvm/mmu.c:2011:17

[  547.956940] index 3 is out of range for type 'kvm_mmu_page *[3]'

[  547.957567] CPU: 0 PID: 21577 Comm: syz-executor Tainted: G      D         4.4.0-next-20160114-sasha-00021-gf1273d1-dirty #2798

[  547.958739]  1ffff1001819be5c 000000002fa0e55b ffff8800c0cdf360 ffffffff83433c4e

[  547.972448]  0000000041b58ab3 ffffffff8f960c38 ffffffff83433b86 ffff8800c0cdf328

[  547.973277]  0000000000000001 000000002fa0e55b ffffffff8feb8440 ffff8800c0cdf3f0

[  547.974102] Call Trace:

[  547.974424] dump_stack (lib/dump_stack.c:52)
[  547.975774] ubsan_epilogue (lib/ubsan.c:165)
[  547.976408] __ubsan_handle_out_of_bounds (lib/ubsan.c:382)
[  547.980877] mmu_zap_unsync_children (arch/x86/kvm/mmu.c:2011 arch/x86/kvm/mmu.c:2272)
[  548.014193] kvm_mmu_prepare_zap_page (arch/x86/kvm/mmu.c:2294)
[  548.018228] kvm_mmu_invalidate_zap_all_pages (arch/x86/kvm/mmu.c:4737 arch/x86/kvm/mmu.c:4776)
[  548.018994] kvm_arch_flush_shadow_all (arch/x86/kvm/x86.c:8050)
[  548.019696] kvm_mmu_notifier_release (include/linux/rcupdate.h:495 include/linux/srcu.h:237 arch/x86/kvm/../../../virt/kvm/kvm_main.c:447)
[  548.021657] __mmu_notifier_release (mm/mmu_notifier.c:66 (discriminator 19))
[  548.024574] exit_mmap (include/linux/mmu_notifier.h:235 mm/mmap.c:2819)
[  548.058887] mmput (kernel/fork.c:707)
[  548.059572] do_exit (./arch/x86/include/asm/bitops.h:311 include/linux/thread_info.h:92 kernel/exit.c:437 kernel/exit.c:735)
[  548.061733] do_group_exit (include/linux/sched.h:813 kernel/exit.c:861)
[  548.062400] get_signal (kernel/signal.c:2327)
[  548.063063] do_signal (arch/x86/kernel/signal.c:781)
[  548.071394] exit_to_usermode_loop (arch/x86/entry/common.c:247)
[  548.072066] syscall_return_slowpath (arch/x86/entry/common.c:282 arch/x86/entry/common.c:344)
[  548.072824] int_ret_from_sys_call (arch/x86/entry/entry_64.S:282)


Thanks,
Sashagl