1 files changed, 128 insertions, 72 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 42d4fc21263b..a0a36bfa3aa0 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -344,8 +344,24 @@ int __mnt_want_write(struct vfsmount *m)
 	 * incremented count after it has set MNT_WRITE_HOLD.
 	 */
 	smp_mb();
-	while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
-		cpu_relax();
+	might_lock(&mount_lock.lock);
+	while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD) {
+		if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+			cpu_relax();
+		} else {
+			/*
+			 * This prevents priority inversion, if the task
+			 * setting MNT_WRITE_HOLD got preempted on a remote
+			 * CPU, and it prevents life lock if the task setting
+			 * MNT_WRITE_HOLD has a lower priority and is bound to
+			 * the same CPU as the task that is spinning here.
+			 */
+			preempt_enable();
+			lock_mount_hash();
+			unlock_mount_hash();
+			preempt_disable();
+		}
+	}
 	/*
 	 * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
 	 * be set to match its requirements. So we must not load that until
@@ -469,6 +485,24 @@ void mnt_drop_write_file(struct file *file)
 }
 EXPORT_SYMBOL(mnt_drop_write_file);
 
+/**
+ * mnt_hold_writers - prevent write access to the given mount
+ * @mnt: mnt to prevent write access to
+ *
+ * Prevents write access to @mnt if there are no active writers for @mnt.
+ * This function needs to be called and return successfully before changing
+ * properties of @mnt that need to remain stable for callers with write access
+ * to @mnt.
+ *
+ * After this functions has been called successfully callers must pair it with
+ * a call to mnt_unhold_writers() in order to stop preventing write access to
+ * @mnt.
+ *
+ * Context: This function expects lock_mount_hash() to be held serializing
+ *          setting MNT_WRITE_HOLD.
+ * Return: On success 0 is returned.
+ *	   On error, -EBUSY is returned.
+ */
 static inline int mnt_hold_writers(struct mount *mnt)
 {
 	mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
@@ -500,6 +534,18 @@ static inline int mnt_hold_writers(struct mount *mnt)
 	return 0;
 }
 
+/**
+ * mnt_unhold_writers - stop preventing write access to the given mount
+ * @mnt: mnt to stop preventing write access to
+ *
+ * Stop preventing write access to @mnt allowing callers to gain write access
+ * to @mnt again.
+ *
+ * This function can only be called after a successful call to
+ * mnt_hold_writers().
+ *
+ * Context: This function expects lock_mount_hash() to be held.
+ */
 static inline void mnt_unhold_writers(struct mount *mnt)
 {
 	/*
@@ -533,12 +579,9 @@ int sb_prepare_remount_readonly(struct super_block *sb)
 	lock_mount_hash();
 	list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
 		if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
-			mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
-			smp_mb();
-			if (mnt_get_writers(mnt) > 0) {
-				err = -EBUSY;
+			err = mnt_hold_writers(mnt);
+			if (err)
 				break;
-			}
 		}
 	}
 	if (!err && atomic_long_read(&sb->s_remove_count))
@@ -2568,6 +2611,7 @@ static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *
 	struct super_block *sb = mnt->mnt_sb;
 
 	if (!__mnt_is_readonly(mnt) &&
+	   (!(sb->s_iflags & SB_I_TS_EXPIRY_WARNED)) &&
 	   (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
 		char *buf = (char *)__get_free_page(GFP_KERNEL);
 		char *mntpath = buf ? d_path(mountpoint, buf, PAGE_SIZE) : ERR_PTR(-ENOMEM);
@@ -2582,6 +2626,7 @@ static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *
 			tm.tm_year+1900, (unsigned long long)sb->s_time_max);
 
 		free_page((unsigned long)buf);
+		sb->s_iflags |= SB_I_TS_EXPIRY_WARNED;
 	}
 }
 
@@ -3969,46 +4014,57 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 	return 0;
 }
 
-static struct mount *mount_setattr_prepare(struct mount_kattr *kattr,
-					   struct mount *mnt, int *err)
+/**
+ * mnt_allow_writers() - check whether the attribute change allows writers
+ * @kattr: the new mount attributes
+ * @mnt: the mount to which @kattr will be applied
+ *
+ * Check whether thew new mount attributes in @kattr allow concurrent writers.
+ *
+ * Return: true if writers need to be held, false if not
+ */
+static inline bool mnt_allow_writers(const struct mount_kattr *kattr,
+				     const struct mount *mnt)
 {
-	struct mount *m = mnt, *last = NULL;
+	return !(kattr->attr_set & MNT_READONLY) ||
+	       (mnt->mnt.mnt_flags & MNT_READONLY);
+}
 
-	if (!is_mounted(&m->mnt)) {
-		*err = -EINVAL;
-		goto out;
-	}
+static int mount_setattr_prepare(struct mount_kattr *kattr, struct mount *mnt)
+{
+	struct mount *m;
+	int err;
 
-	if (!(mnt_has_parent(m) ? check_mnt(m) : is_anon_ns(m->mnt_ns))) {
-		*err = -EINVAL;
-		goto out;
-	}
+	for (m = mnt; m; m = next_mnt(m, mnt)) {
+		if (!can_change_locked_flags(m, recalc_flags(kattr, m))) {
+			err = -EPERM;
+			break;
+		}
 
-	do {
-		unsigned int flags;
+		err = can_idmap_mount(kattr, m);
+		if (err)
+			break;
 
-		flags = recalc_flags(kattr, m);
-		if (!can_change_locked_flags(m, flags)) {
-			*err = -EPERM;
-			goto out;
+		if (!mnt_allow_writers(kattr, m)) {
+			err = mnt_hold_writers(m);
+			if (err)
+				break;
 		}
 
-		*err = can_idmap_mount(kattr, m);
-		if (*err)
-			goto out;
+		if (!kattr->recurse)
+			return 0;
+	}
 
-		last = m;
+	if (err) {
+		struct mount *p;
 
-		if ((kattr->attr_set & MNT_READONLY) &&
-		    !(m->mnt.mnt_flags & MNT_READONLY)) {
-			*err = mnt_hold_writers(m);
-			if (*err)
-				goto out;
+		for (p = mnt; p != m; p = next_mnt(p, mnt)) {
+			/* If we had to hold writers unblock them. */
+			if (p->mnt.mnt_flags & MNT_WRITE_HOLD)
+				mnt_unhold_writers(p);
 		}
-	} while (kattr->recurse && (m = next_mnt(m, mnt)));
-
-out:
-	return last;
+	}
+	return err;
 }
 
 static void do_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
@@ -4036,48 +4092,32 @@ static void do_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 		put_user_ns(old_mnt_userns);
 }
 
-static void mount_setattr_commit(struct mount_kattr *kattr,
-				 struct mount *mnt, struct mount *last,
-				 int err)
+static void mount_setattr_commit(struct mount_kattr *kattr, struct mount *mnt)
 {
-	struct mount *m = mnt;
+	struct mount *m;
 
-	do {
-		if (!err) {
-			unsigned int flags;
+	for (m = mnt; m; m = next_mnt(m, mnt)) {
+		unsigned int flags;
 
-			do_idmap_mount(kattr, m);
-			flags = recalc_flags(kattr, m);
-			WRITE_ONCE(m->mnt.mnt_flags, flags);
-		}
+		do_idmap_mount(kattr, m);
+		flags = recalc_flags(kattr, m);
+		WRITE_ONCE(m->mnt.mnt_flags, flags);
 
-		/*
-		 * We either set MNT_READONLY above so make it visible
-		 * before ~MNT_WRITE_HOLD or we failed to recursively
-		 * apply mount options.
-		 */
-		if ((kattr->attr_set & MNT_READONLY) &&
-		    (m->mnt.mnt_flags & MNT_WRITE_HOLD))
+		/* If we had to hold writers unblock them. */
+		if (m->mnt.mnt_flags & MNT_WRITE_HOLD)
 			mnt_unhold_writers(m);
 
-		if (!err && kattr->propagation)
+		if (kattr->propagation)
 			change_mnt_propagation(m, kattr->propagation);
-
-		/*
-		 * On failure, only cleanup until we found the first mount
-		 * we failed to handle.
-		 */
-		if (err && m == last)
+		if (!kattr->recurse)
 			break;
-	} while (kattr->recurse && (m = next_mnt(m, mnt)));
-
-	if (!err)
-		touch_mnt_namespace(mnt->mnt_ns);
+	}
+	touch_mnt_namespace(mnt->mnt_ns);
 }
 
 static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
 {
-	struct mount *mnt = real_mount(path->mnt), *last = NULL;
+	struct mount *mnt = real_mount(path->mnt);
 	int err = 0;
 
 	if (path->dentry != mnt->mnt.mnt_root)
@@ -4098,16 +4138,32 @@ static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
 		}
 	}
 
+	err = -EINVAL;
 	lock_mount_hash();
 
+	/* Ensure that this isn't anything purely vfs internal. */
+	if (!is_mounted(&mnt->mnt))
+		goto out;
+
+	/*
+	 * If this is an attached mount make sure it's located in the callers
+	 * mount namespace. If it's not don't let the caller interact with it.
+	 * If this is a detached mount make sure it has an anonymous mount
+	 * namespace attached to it, i.e. we've created it via OPEN_TREE_CLONE.
+	 */
+	if (!(mnt_has_parent(mnt) ? check_mnt(mnt) : is_anon_ns(mnt->mnt_ns)))
+		goto out;
+
 	/*
-	 * Get the mount tree in a shape where we can change mount
-	 * properties without failure.
+	 * First, we get the mount tree in a shape where we can change mount
+	 * properties without failure. If we succeeded to do so we commit all
+	 * changes and if we failed we clean up.
 	 */
-	last = mount_setattr_prepare(kattr, mnt, &err);
-	if (last) /* Commit all changes or revert to the old state. */
-		mount_setattr_commit(kattr, mnt, last, err);
+	err = mount_setattr_prepare(kattr, mnt);
+	if (!err)
+		mount_setattr_commit(kattr, mnt);
 
+out:
 	unlock_mount_hash();
 
 	if (kattr->propagation) {