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Date:   Mon, 21 Mar 2022 14:42:36 +0100
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From:   "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
Subject: Re: [syzbot] WARNING in kvm_mmu_notifier_invalidate_range_start (2)
To:     Paolo Bonzini <pbonzini@redhat.com>
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On 21.03.2022 12:01, Paolo Bonzini wrote:
> On 3/21/22 11:25, syzbot wrote:
>> syz repro:      https://urldefense.com/v3/__https://syzkaller.appspot.com/x/repro.syz?x=12a2d0a9700000__;!!ACWV5N9M2RV99hQ!bJGc10O9acwj6GeDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7ye0Uq2ZA$ C reproducer:   https://urldefense.com/v3/__https://syzkaller.appspot.com/x/repro.c?x=13d34fd9700000__;!!ACWV5N9M2RV99hQ!bJGc10O9acwj6GeDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7xoEv26SQ$
>> The issue was bisected to:
>>
>> commit ed922739c9199bf515a3e7fec3e319ce1edeef2a
>> Author: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
>> Date:   Mon Dec 6 19:54:28 2021 +0000
>>
>>      KVM: Use interval tree to do fast hva lookup in memslots
>>
>> bisection log:  https://urldefense.com/v3/__https://syzkaller.appspot.com/x/bisect.txt?x=142aa59d700000__;!!ACWV5N9M2RV99hQ!bJGc10O9acwj6GeDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7xEhtZ-FQ$ final oops:     https://urldefense.com/v3/__https://syzkaller.appspot.com/x/report.txt?x=162aa59d700000__;!!ACWV5N9M2RV99hQ!bJGc10O9acwj6GeDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7zcn2K3LQ$ console output: https://urldefense.com/v3/__https://syzkaller.appspot.com/x/log.txt?x=122aa59d700000__;!!ACWV5N9M2RV99hQ!bJGc10O9acwj6GeDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7wzducgVQ$ 
> 
> It bisects here just because the patch introduces the warning; the issue is a mmu_notifier_invalidate_range_start with an empty range.  The offending system call
> 
> mremap(&(0x7f000000d000/0x2000)=nil, 0xfffffffffffffe74, 0x1000, 0x3, &(0x7f0000007000/0x1000)=nil)
> 
> really means old_len == 0 (it's page-aligned at the beginning of sys_mremap), and flags includes MREMAP_FIXED so it goes down to mremap_to and from there to move_page_tables.  No function on this path attempts to special case old_len == 0, the immediate fix would be
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 002eec83e91e..0e175aef536e 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -486,6 +486,9 @@ unsigned long move_page_tables(struct vm_area_struct
>       pmd_t *old_pmd, *new_pmd;
>       pud_t *old_pud, *new_pud;
> 
> +    if (!len)
> +        return 0;
> +
>       old_end = old_addr + len;
>       flush_cache_range(vma, old_addr, old_end);
> 
> but there are several other ways to fix this elsewhere in the call chain:
> 
> - check for old_len == 0 somewhere in mremap_to
> 
> - skip the call in __mmu_notifier_invalidate_range_start and __mmu_notifier_invalidate_range_end, if people agree not to play whack-a-mole with the callers of mmu_notifier_invalidate_range_*.
> 
> - remove the warning in KVM

This probably depends whether it is actually legal to call MMU notifiers
with a zero range, the first time this warning triggered it was the caller
that was fixed [1].

By the way, the warning-on-zero-range was added during memslots patch set
review process [2], but I think it ultimately does make sense.

> Thanks,
> 
> Paolo
> 

Thanks,
Maciej

[1]: https://lore.kernel.org/kvm/20211228234257.1926057-1-seanjc@google.com/
[2]: https://lore.kernel.org/kvm/YKWaFwgMNSaQQuQP@google.com/