[PATCH v1 1/1] KVM: s390: Fix gmap_helper_zap_one_page() again

[PATCH v1 1/1] KVM: s390: Fix gmap_helper_zap_one_page() again

[Claudio Imbrenda]

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@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.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

Re: [PATCH v1 1/1] KVM: s390: Fix gmap_helper_zap_one_page() again

[Marc Hartmayer]

On Thu, Nov 06, 2025 at 04:25 PM +0100, Claudio Imbrenda <imbrenda@linux.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@linux.ibm.com>
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.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@linux.ibm.com>