[PATCH] s390/mm: add missing secure storage access fixups for donated memory

[PATCH] s390/mm: add missing secure storage access fixups for donated memory

[Janosch Frank]

There are special cases where secure storage access exceptions happen
in a kernel context for pages that don't have the PG_arch_1 bit
set. That bit is set for non-exported guest secure storage (memory)
but is absent on storage donated to the Ultravisor since the kernel
isn't allowed to export donated pages.

Prior to this patch we would try to export the page by calling
arch_make_folio_accessible() which would instantly return since the
arch bit is absent signifying that the page was already exported and
no further action is necessary. This leads to secure storage access
exception loops which can never be resolved.

With this patch we unconditionally try to export and if that fails we
fixup.

Fixes: 084ea4d611a3 ("s390/mm: add (non)secure page access exceptions handlers")
Reported-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
---
 arch/s390/mm/fault.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index a52aa7a99b6b..71bad4257aab 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -441,10 +441,15 @@ void do_secure_storage_access(struct pt_regs *regs)
 		folio = phys_to_folio(addr);
 		if (unlikely(!folio_try_get(folio)))
 			return;
-		rc = arch_make_folio_accessible(folio);
+		rc = uv_convert_from_secure(folio_to_phys(folio));
 		folio_put(folio);
+		/*
+		 * There are some valid fixup types for kernel
+		 * accesses to donated secure memory. zeropad is one
+		 * of them.
+		 */
 		if (rc)
-			BUG();
+			return handle_fault_error(regs, 0);
 	} else {
 		if (faulthandler_disabled())
 			return handle_fault_error_nolock(regs, 0);
-- 
2.51.0

Re: [PATCH] s390/mm: add missing secure storage access fixups for donated memory

[Heiko Carstens]

On Mon, Mar 09, 2026 at 12:53:15PM +0000, Janosch Frank wrote:
> There are special cases where secure storage access exceptions happen
> in a kernel context for pages that don't have the PG_arch_1 bit
> set. That bit is set for non-exported guest secure storage (memory)
> but is absent on storage donated to the Ultravisor since the kernel
> isn't allowed to export donated pages.
> 
> Prior to this patch we would try to export the page by calling
> arch_make_folio_accessible() which would instantly return since the
> arch bit is absent signifying that the page was already exported and
> no further action is necessary. This leads to secure storage access
> exception loops which can never be resolved.
> 
> With this patch we unconditionally try to export and if that fails we
> fixup.
> 
> Fixes: 084ea4d611a3 ("s390/mm: add (non)secure page access exceptions handlers")
> Reported-by: Heiko Carstens <hca@linux.ibm.com>
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>  arch/s390/mm/fault.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index a52aa7a99b6b..71bad4257aab 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -441,10 +441,15 @@ void do_secure_storage_access(struct pt_regs *regs)
>  		folio = phys_to_folio(addr);
>  		if (unlikely(!folio_try_get(folio)))
>  			return;
> -		rc = arch_make_folio_accessible(folio);
> +		rc = uv_convert_from_secure(folio_to_phys(folio));
>  		folio_put(folio);
> +		/*
> +		 * There are some valid fixup types for kernel
> +		 * accesses to donated secure memory. zeropad is one
> +		 * of them.
> +		 */
>  		if (rc)
> -			BUG();
> +			return handle_fault_error(regs, 0);

This context doesn't hold mmlock, so it should be:

			return handle_fault_error_nolock(regs, 0);

And, yes, you can blame me, since I proposed the wrong call off-list.