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Date:   Wed, 8 Apr 2020 09:04:36 +0200
From:   Cornelia Huck <cohuck@redhat.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Hildenbrand <david@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        syzbot+d889b59b2bb87d4047a2@syzkaller.appspotmail.com
Subject: Re: [PATCH 1/2] KVM: Check validity of resolved slot when searching
 memslots
Message-ID: <20200408090436.2bd1f303.cohuck@redhat.com>
In-Reply-To: <20200408064059.8957-2-sean.j.christopherson@intel.com>
References: <20200408064059.8957-1-sean.j.christopherson@intel.com>
        <20200408064059.8957-2-sean.j.christopherson@intel.com>
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On Tue,  7 Apr 2020 23:40:58 -0700
Sean Christopherson <sean.j.christopherson@intel.com> wrote:

> Check that the resolved slot (somewhat confusingly named 'start') is a
> valid/allocated slot before doing the final comparison to see if the
> specified gfn resides in the associated slot.  The resolved slot can be
> invalid if the binary search loop terminated because the search index
> was incremented beyond the number of used slots.
> 
> This bug has existed since the binary search algorithm was introduced,
> but went unnoticed because KVM statically allocated memory for the max
> number of slots, i.e. the access would only be truly out-of-bounds if
> all possible slots were allocated and the specified gfn was less than
> the base of the lowest memslot.  Commit 36947254e5f98 ("KVM: Dynamically
> size memslot array based on number of used slots") eliminated the "all
> possible slots allocated" condition and made the bug embarrasingly easy
> to hit.
> 
> Fixes: 9c1a5d38780e6 ("kvm: optimize GFN to memslot lookup with large slots amount")
> Reported-by: syzbot+d889b59b2bb87d4047a2@syzkaller.appspotmail.com
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  include/linux/kvm_host.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 6d58beb65454..01276e3d01b9 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1048,7 +1048,7 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn)
>  			start = slot + 1;
>  	}
>  
> -	if (gfn >= memslots[start].base_gfn &&
> +	if (start < slots->used_slots && gfn >= memslots[start].base_gfn &&
>  	    gfn < memslots[start].base_gfn + memslots[start].npages) {
>  		atomic_set(&slots->lru_slot, start);
>  		return &memslots[start];

Reviewed-by: Cornelia Huck <cohuck@redhat.com>