s390/mm: Add missing secure storage access fixups for donated memory

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: 084ea4d ("s390/mm: add (non)secure page access exceptions handlers")
Reported-by: Heiko Carstens <[email protected]>
Suggested-by: Heiko Carstens <[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Tested-by: Christian Borntraeger <[email protected]>
Signed-off-by: Janosch Frank <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>

Author: frankjaa

arch/s390/mm/fault.c

@@ -441,10 +441,17 @@ 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));
+		if (!rc)
+			clear_bit(PG_arch_1, &folio->flags.f);
 		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_nolock(regs, 0);
 	} else {
 		if (faulthandler_disabled())
 			return handle_fault_error_nolock(regs, 0);