Subject: [RFC PATCH 1/2] libnvdimm: Add prctl control for disabling synchronous fault support.

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 29 May 2020 11:11:39 +0530

With POWER10, architecture is adding new pmem flush and sync instructions.
The kernel should prevent the usage of MAP_SYNC if applications are not using
the new instructions on newer hardware.

This patch adds a prctl option MAP_SYNC_ENABLE that can be used to enable
the usage of MAP_SYNC. The kernel config option is added to allow the user
to control whether MAP_SYNC should be enabled by default or not.

Signed-off-by: Aneesh Kumar K.V <[email-masked]>
---
 include/linux/sched/coredump.h | 13 ++++++++++---
 include/uapi/linux/prctl.h     |  3 +++
 kernel/fork.c                  |  8 +++++++-
 kernel/sys.c                   | 18 ++++++++++++++++++
 mm/Kconfig                     |  3 +++
 mm/mmap.c                      |  4 ++++
 6 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index ecdc6542070f..9ba6b3d5f991 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -72,9 +72,16 @@ static inline int get_dumpable(struct mm_struct *mm)
 #define MMF_DISABLE_THP		24	/* disable THP for all VMAs */
 #define MMF_OOM_VICTIM		25	/* mm is the oom victim */
 #define MMF_OOM_REAP_QUEUED	26	/* mm was queued for oom_reaper */
-#define MMF_DISABLE_THP_MASK	(1 << MMF_DISABLE_THP)
+#define MMF_DISABLE_MAP_SYNC	27	/* disable THP for all VMAs */
+#define MMF_DISABLE_THP_MASK		(1 << MMF_DISABLE_THP)
+#define MMF_DISABLE_MAP_SYNC_MASK	(1 << MMF_DISABLE_MAP_SYNC)
 
-#define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
-				 MMF_DISABLE_THP_MASK)
+#define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK | \
+			MMF_DISABLE_THP_MASK | MMF_DISABLE_MAP_SYNC_MASK)
+
+static inline bool map_sync_enabled(struct mm_struct *mm)
+{
+	return !(mm->flags & MMF_DISABLE_MAP_SYNC_MASK);
+}
 
 #endif /* _LINUX_SCHED_COREDUMP_H */
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f8131e36..ee4cde32d5cf 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -238,4 +238,7 @@ struct prctl_mm_map {
 #define PR_SET_IO_FLUSHER		57
 #define PR_GET_IO_FLUSHER		58
 
+#define PR_SET_MAP_SYNC_ENABLE		59
+#define PR_GET_MAP_SYNC_ENABLE		60
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index 8c700f881d92..d5a9a363e81e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -963,6 +963,12 @@ __cacheline_aligned_in_smp DEFINE_SPINLOCK(mmlist_lock);
 
 static unsigned long default_dump_filter = MMF_DUMP_FILTER_DEFAULT;
 
+#ifdef CONFIG_ARCH_MAP_SYNC_DISABLE
+unsigned long default_map_sync_mask = MMF_DISABLE_MAP_SYNC_MASK;
+#else
+unsigned long default_map_sync_mask = 0;
+#endif
+
 static int __init coredump_filter_setup(char *s)
 {
 	default_dump_filter =
@@ -1039,7 +1045,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 		mm->flags = current->mm->flags & MMF_INIT_MASK;
 		mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
 	} else {
-		mm->flags = default_dump_filter;
+		mm->flags = default_dump_filter | default_map_sync_mask;
 		mm->def_flags = 0;
 	}
 
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..f6127cf4128b 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2450,6 +2450,24 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			clear_bit(MMF_DISABLE_THP, &me->mm->flags);
 		up_write(&me->mm->mmap_sem);
 		break;
+
+	case PR_GET_MAP_SYNC_ENABLE:
+		if (arg2 || arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = !test_bit(MMF_DISABLE_MAP_SYNC, &me->mm->flags);
+		break;
+	case PR_SET_MAP_SYNC_ENABLE:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		if (down_write_killable(&me->mm->mmap_sem))
+			return -EINTR;
+		if (arg2)
+			clear_bit(MMF_DISABLE_MAP_SYNC, &me->mm->flags);
+		else
+			set_bit(MMF_DISABLE_MAP_SYNC, &me->mm->flags);
+		up_write(&me->mm->mmap_sem);
+		break;
+
 	case PR_MPX_ENABLE_MANAGEMENT:
 	case PR_MPX_DISABLE_MANAGEMENT:
 		/* No longer implemented: */
diff --git a/mm/Kconfig b/mm/Kconfig
index c1acc34c1c35..38fd7cfbfca8 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -867,4 +867,7 @@ config ARCH_HAS_HUGEPD
 config MAPPING_DIRTY_HELPERS
         bool
 
+config ARCH_MAP_SYNC_DISABLE
+	bool
+
 endmenu
diff --git a/mm/mmap.c b/mm/mmap.c
index f609e9ec4a25..613e5894f178 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1464,6 +1464,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 		case MAP_SHARED_VALIDATE:
 			if (flags & ~flags_mask)
 				return -EOPNOTSUPP;
+
+			if ((flags & MAP_SYNC)  && !map_sync_enabled(mm))
+				return -EOPNOTSUPP;
+
 			if (prot & PROT_WRITE) {
 				if (!(file->f_mode & FMODE_WRITE))
 					return -EACCES;
-- 
2.26.2