linux-kernel - Re: [PATCH v1 3/5] KVM: s390: vsie: Fix possible race when shadowing region 3 tables

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Date:   Mon, 6 Apr 2020 15:47:32 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Janosch Frank <frankja@...ux.ibm.com>
Subject: Re: [PATCH v1 3/5] KVM: s390: vsie: Fix possible race when shadowing
 region 3 tables



On 02.04.20 20:48, David Hildenbrand wrote:
> We have to properly retry again by returning -EINVAL immediately in case
> somebody else instantiated the table concurrently. We missed to add the
> goto in this function only. The code now matches the other, similar
> shadowing functions.
> 
> We are overwriting an existing region 2 table entry. All allocated pages
> are added to the crst_list to be freed later, so they are not lost
> forever. However, when unshadowing the region 2 table, we wouldn't trigger
> unshadowing of the original shadowed region 3 table that we replaced. It
> would get unshadowed when the original region 3 table is modified. As it's
> not connected to the page table hierarchy anymore, it's not going to get
> used anymore. However, for a limited time, this page table will stick
> around, so it's in some sense a temporary memory leak.
> 
> Identified by manual code inspection. I don't think this classifies as
> stable material.
> 
> Fixes: 998f637cc4b9 ("s390/mm: avoid races on region/segment/page table shadowing")
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  arch/s390/mm/gmap.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index f3dbc5bdde50..fd32ab566f57 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -1844,6 +1844,7 @@ int gmap_shadow_r3t(struct gmap *sg, unsigned long saddr, unsigned long r3t,
>  		goto out_free;
>  	} else if (*table & _REGION_ENTRY_ORIGIN) {
>  		rc = -EAGAIN;		/* Race with shadow */
> +		goto out_free;
>  	}
>  	crst_table_init(s_r3t, _REGION3_ENTRY_EMPTY);
>  	/* mark as invalid as long as the parent table is not protected */

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