linux-kernel - Re: [PATCH v2 3/5] KVM: s390: vsie: Fix possible race when shadowing region 3 tables

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Date:   Tue, 7 Apr 2020 13:05:22 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     kvm@...r.kernel.org, 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>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v2 3/5] KVM: s390: vsie: Fix possible race when
 shadowing region 3 tables

On Fri,  3 Apr 2020 17:30:48 +0200
David Hildenbrand <david@...hat.com> 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 b93dd54b234a..24ef30fb0833 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: Claudio Imbrenda <imbrenda@...ux.ibm.com>