linux-kernel - Re: [PATCH v1 1/1] KVM: s390: Fix gmap_helper_zap_one

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Message-ID: <87346l7v9i.fsf@linux.ibm.com>
Date: Mon, 10 Nov 2025 16:38:33 +0100
From: "Marc Hartmayer" <mhartmay@...ux.ibm.com>
To: Claudio Imbrenda <imbrenda@...ux.ibm.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc: linux-s390@...r.kernel.org, borntraeger@...ibm.com, frankja@...ux.ibm.com,
        nrb@...ux.ibm.com, nsg@...ux.ibm.com, seiden@...ux.ibm.com,
        gra@...ux.ibm.com, schlameuss@...ux.ibm.com, hca@...ux.ibm.com,
        svens@...ux.ibm.com, agordeev@...ux.ibm.com, gor@...ux.ibm.com,
        ggala@...ux.ibm.com, david@...hat.com
Subject: Re: [PATCH v1 1/1] KVM: s390: Fix gmap_helper_zap_one_page() again

On Thu, Nov 06, 2025 at 04:25 PM +0100, Claudio Imbrenda <imbrenda@...ux.ibm.com> wrote:
> A few checks were missing in gmap_helper_zap_one_page(), which can lead
> to memory corruption in the guest under specific circumstances.
>
> Add the missing checks.
>
> Fixes: 5deafa27d9ae ("KVM: s390: Fix to clear PTE when discarding a swapped page")
> Reported-by: Marc Hartmayer <mhartmay@...ux.ibm.com>
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> ---
>  arch/s390/mm/gmap_helpers.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/s390/mm/gmap_helpers.c b/arch/s390/mm/gmap_helpers.c
> index d4c3c36855e2..38a2d82cd88a 100644
> --- a/arch/s390/mm/gmap_helpers.c
> +++ b/arch/s390/mm/gmap_helpers.c
> @@ -47,6 +47,7 @@ static void ptep_zap_swap_entry(struct mm_struct *mm, swp_entry_t entry)
>  void gmap_helper_zap_one_page(struct mm_struct *mm, unsigned long vmaddr)
>  {
>  	struct vm_area_struct *vma;
> +	unsigned long pgstev;
>  	spinlock_t *ptl;
>  	pgste_t pgste;
>  	pte_t *ptep;
> @@ -65,9 +66,13 @@ void gmap_helper_zap_one_page(struct mm_struct *mm, unsigned long vmaddr)
>  	if (pte_swap(*ptep)) {
>  		preempt_disable();
>  		pgste = pgste_get_lock(ptep);
> +		pgstev = pgste_val(pgste);
>  
> -		ptep_zap_swap_entry(mm, pte_to_swp_entry(*ptep));
> -		pte_clear(mm, vmaddr, ptep);
> +		if ((pgstev & _PGSTE_GPS_USAGE_MASK) == _PGSTE_GPS_USAGE_UNUSED ||
> +		    (pgstev & _PGSTE_GPS_ZERO)) {
> +			ptep_zap_swap_entry(mm, pte_to_swp_entry(*ptep));
> +			pte_clear(mm, vmaddr, ptep);
> +		}
>  
>  		pgste_set_unlock(ptep, pgste);
>  		preempt_enable();
> -- 
> 2.51.1

Thanks for the fix.

Tested-by: Marc Hartmayer <mhartmay@...ux.ibm.com>