Re: [PATCH v1 2/2] KVM: s390: fix KVM_S390_GET_CMMA_BITS for GFNs in memslot holes

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On Wed,  8 Feb 2023 15:48:26 +0100
Nico Boehr <nrb@xxxxxxxxxxxxx> wrote:

> The KVM_S390_GET_CMMA_BITS ioctl may return incorrect values when userspace
> specifies a start_gfn outside of memslots.
> 
> This can occur when a VM has multiple memslots with a hole in between:
> 
> +-----+----------+--------+--------+
> | ... | Slot N-1 | <hole> | Slot N |
> +-----+----------+--------+--------+
>       ^          ^        ^        ^
>       |          |        |        |
> GFN   A          A+B      |        |
>                           A+B+C    |
> 			           A+B+C+D
> 
> When userspace specifies a GFN in [A+B, A+B+C), it would expect to get the
> CMMA values of the first dirty page in Slot N. However, userspace may get a
> start_gfn of A+B+C+D with a count of 0, hence completely skipping over any
> dirty pages in slot N.
> 
> The error is in kvm_s390_next_dirty_cmma(), which assumes
> gfn_to_memslot_approx() will return the memslot _below_ the specified GFN
> when the specified GFN lies outside a memslot. In reality it may return
> either the memslot below or above the specified GFN.
> 
> When a memslot above the specified GFN is returned this happens:
> 
> - ofs is calculated, but since the memslot's base_gfn is larger than the
>   specified cur_gfn, ofs will underflow to a huge number.
> - ofs is passed to find_next_bit(). Since ofs will exceed the memslot's
>   number of pages, the number of pages in the memslot is returned,
>   completely skipping over all bits in the memslot userspace would be
>   interested in.
> 
> Fix this by resetting ofs to zero when a memslot _above_ cur_gfn is
> returned (cur_gfn < ms->base_gfn).
> 
> Signed-off-by: Nico Boehr <nrb@xxxxxxxxxxxxx>

Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>

> ---
>  arch/s390/kvm/kvm-s390.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index e4890e04b210..a171a66681b4 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2158,6 +2158,10 @@ static unsigned long kvm_s390_next_dirty_cmma(struct kvm_memslots *slots,
>  		ms = container_of(mnode, struct kvm_memory_slot, gfn_node[slots->node_idx]);
>  		ofs = 0;
>  	}
> +
> +	if (cur_gfn < ms->base_gfn)
> +		ofs = 0;
> +
>  	ofs = find_next_bit(kvm_second_dirty_bitmap(ms), ms->npages, ofs);
>  	while (ofs >= ms->npages && (mnode = rb_next(mnode))) {
>  		ms = container_of(mnode, struct kvm_memory_slot, gfn_node[slots->node_idx]);




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