linux-kernel - Re: [PATCH v2 1/5] KVM: s390: vsie: Fix region 1 ASCE sanity shadow address checks

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Date:   Fri, 3 Apr 2020 19:56:31 +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>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/5] KVM: s390: vsie: Fix region 1 ASCE sanity shadow
 address checks



On 03.04.20 17:30, David Hildenbrand wrote:
> In case we have a region 1 ASCE, our shadow/g3 address can have any value.
> Unfortunately, (-1UL << 64) is undefined and triggers sometimes,
> rejecting valid shadow addresses when trying to walk our shadow table
> hierarchy.

I thin the range of the addresses do not matter.
Took me a while to understand maybe rephrase that:

In case we have a region 1 the following calculation 
(31 + ((gmap->asce & _ASCE_TYPE_MASK) >> 2)*11)
results in 64. As shifts beyond the size are undefined the compiler is free to use
instructions like sllg. sllg will only use 6 bits of the shift value (here 64)
resulting in no shift at all. That means that ALL addresses will be rejected.

With that this makes sense. 

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


> 
> The result is that the prefix cannot get mapped and will loop basically
> forever trying to map it (-EAGAIN loop).
> 
> After all, the broken check is only a sanity check, our table shadowing
> code in kvm_s390_shadow_tables() already checks these conditions, injecting
> proper translation exceptions. Turn it into a WARN_ON_ONCE().
> 
> Fixes: 4be130a08420 ("s390/mm: add shadow gmap support")
> Tested-by: Janosch Frank <frankja@...ux.ibm.com>
> Reported-by: Janosch Frank <frankja@...ux.ibm.com>
> Cc: <stable@...r.kernel.org> # v4.8+
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  arch/s390/mm/gmap.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 2fbece47ef6f..b93dd54b234a 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -787,14 +787,18 @@ static void gmap_call_notifier(struct gmap *gmap, unsigned long start,
>  static inline unsigned long *gmap_table_walk(struct gmap *gmap,
>  					     unsigned long gaddr, int level)
>  {
> +	const int asce_type = gmap->asce & _ASCE_TYPE_MASK;
>  	unsigned long *table;
>  
>  	if ((gmap->asce & _ASCE_TYPE_MASK) + 4 < (level * 4))
>  		return NULL;
>  	if (gmap_is_shadow(gmap) && gmap->removed)
>  		return NULL;
> -	if (gaddr & (-1UL << (31 + ((gmap->asce & _ASCE_TYPE_MASK) >> 2)*11)))
> +
> +	if (WARN_ON_ONCE(asce_type != _ASCE_TYPE_REGION1 &&
> +			 gaddr & (-1UL << (31 + (asce_type >> 2) * 11))))
>  		return NULL;
> +
>  	table = gmap->table;
>  	switch (gmap->asce & _ASCE_TYPE_MASK) {
>  	case _ASCE_TYPE_REGION1:
>