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Subject: Re: [PATCH 1/7] KVM: Fix out of range accesses to memslots
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Qian Cai <cai@lca.pw>,
        Peter Xu <peterx@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>
References: <20200320205546.2396-1-sean.j.christopherson@intel.com>
 <20200320205546.2396-2-sean.j.christopherson@intel.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Tue, 24 Mar 2020 08:12:59 +0100
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On 20.03.20 21:55, Sean Christopherson wrote:
> Reset the LRU slot if it becomes invalid when deleting a memslot to fix
> an out-of-bounds/use-after-free access when searching through memslots.
> 
> Explicitly check for there being no used slots in search_memslots(), and
> in the caller of s390's approximation variant.
> 
> Fixes: 36947254e5f9 ("KVM: Dynamically size memslot array based on number of used slots")
> Reported-by: Qian Cai <cai@lca.pw>
> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/s390/kvm/kvm-s390.c | 3 +++
>  include/linux/kvm_host.h | 3 +++
>  virt/kvm/kvm_main.c      | 3 +++
>  3 files changed, 9 insertions(+)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 807ed6d722dd..cb15fdda1fee 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2002,6 +2002,9 @@ static int kvm_s390_get_cmma(struct kvm *kvm, struct kvm_s390_cmma_log *args,

Adding Claudio, but
>  	struct kvm_memslots *slots = kvm_memslots(kvm);
>  	struct kvm_memory_slot *ms;
>  
> +	if (unlikely(!slots->used_slots))
> +		return 0;
> +

this looks sane and like the right fix.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>

>  	cur_gfn = kvm_s390_next_dirty_cmma(slots, args->start_gfn);
>  	ms = gfn_to_memslot(kvm, cur_gfn);
>  	args->count = 0;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 35bc52e187a2..b19dee4ed7d9 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1032,6 +1032,9 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn)
>  	int slot = atomic_read(&slots->lru_slot);
>  	struct kvm_memory_slot *memslots = slots->memslots;
>  
> +	if (unlikely(!slots->used_slots))
> +		return NULL;
> +
>  	if (gfn >= memslots[slot].base_gfn &&
>  	    gfn < memslots[slot].base_gfn + memslots[slot].npages)
>  		return &memslots[slot];
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 28eae681859f..f744bc603c53 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -882,6 +882,9 @@ static inline void kvm_memslot_delete(struct kvm_memslots *slots,
>  
>  	slots->used_slots--;
>  
> +	if (atomic_read(&slots->lru_slot) >= slots->used_slots)
> +		atomic_set(&slots->lru_slot, 0);
> +
>  	for (i = slots->id_to_index[memslot->id]; i < slots->used_slots; i++) {
>  		mslots[i] = mslots[i + 1];
>  		slots->id_to_index[mslots[i].id] = i;
>