[PATCH v4 3/5] liveupdate: block session mutations during reboot

[Pasha Tatashin]

During the reboot() syscall, user processes may still be running
concurrently and attempting to mutate sessions (e.g., creating,
retrieving, or releasing sessions). To prevent this, introduce
luo_session_serialize_rwsem to synchronize mutations with the
serialization process.

All session mutation operations (create, retrieve, release, ioctl) take
the read lock. The serialization process (luo_session_serialize) takes
the write lock and holds it indefinitely on success. This effectively
freezes the LUO session subsystem during the transition to the new
kernel. If serialization fails, the lock is released to allow recovery.

Fixes: 0153094d03df ("liveupdate: luo_session: add sessions support")
Reported-by: Oskar Gerlicz Kowalczuk <oskar at gerlicz.space>
Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>
---
 kernel/liveupdate/luo_session.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
index 59b37d17db6b..6099213275d8 100644
--- a/kernel/liveupdate/luo_session.c
+++ b/kernel/liveupdate/luo_session.c
@@ -75,6 +75,13 @@
 		sizeof(struct luo_session_header_ser)) /		\
 		sizeof(struct luo_session_ser))
 
+/*
+ * Protects session mutations during serialization. All session mutation
+ * operations must hold the read lock. The serialization process holds the write
+ * lock indefinitely on success to block all concurrent and future mutations.
+ */
+static DECLARE_RWSEM(luo_session_serialize_rwsem);
+
 /**
  * struct luo_session_header - Header struct for managing LUO sessions.
  * @count:      The number of sessions currently tracked in the @list.
@@ -205,6 +212,7 @@ static int luo_session_release(struct inode *inodep, struct file *filep)
 	struct luo_session *session = filep->private_data;
 	struct luo_session_header *sh;
 
+	guard(rwsem_read)(&luo_session_serialize_rwsem);
 	/* If retrieved is set, it means this session is from incoming list */
 	if (session->retrieved) {
 		int err = luo_session_finish_one(session);
@@ -354,6 +362,7 @@ static long luo_session_ioctl(struct file *filep, unsigned int cmd,
 	if (ret)
 		return ret;
 
+	guard(rwsem_read)(&luo_session_serialize_rwsem);
 	return op->execute(session, &ucmd);
 }
 
@@ -385,9 +394,12 @@ int luo_session_create(const char *name, struct file **filep)
 	struct luo_session *session;
 	int err;
 
+	down_read(&luo_session_serialize_rwsem);
 	session = luo_session_alloc(name);
-	if (IS_ERR(session))
-		return PTR_ERR(session);
+	if (IS_ERR(session)) {
+		err = PTR_ERR(session);
+		goto err_unlock;
+	}
 
 	err = luo_session_insert(&luo_session_global.outgoing, session);
 	if (err)
@@ -398,12 +410,16 @@ int luo_session_create(const char *name, struct file **filep)
 	if (err)
 		goto err_remove;
 
+	up_read(&luo_session_serialize_rwsem);
+
 	return 0;
 
 err_remove:
 	luo_session_remove(&luo_session_global.outgoing, session);
 err_free:
 	luo_session_free(session);
+err_unlock:
+	up_read(&luo_session_serialize_rwsem);
 
 	return err;
 }
@@ -415,6 +431,7 @@ int luo_session_retrieve(const char *name, struct file **filep)
 	struct luo_session *it;
 	int err;
 
+	guard(rwsem_read)(&luo_session_serialize_rwsem);
 	guard(rwsem_read)(&sh->rwsem);
 	list_for_each_entry(it, &sh->list, list) {
 		if (!strncmp(it->name, name, sizeof(it->name))) {
@@ -582,7 +599,8 @@ int luo_session_serialize(void)
 	int i = 0;
 	int err;
 
-	guard(rwsem_write)(&sh->rwsem);
+	down_write(&luo_session_serialize_rwsem);
+	down_write(&sh->rwsem);
 	list_for_each_entry(session, &sh->list, list) {
 		err = luo_session_freeze_one(session, &sh->ser[i]);
 		if (err)
@@ -593,6 +611,7 @@ int luo_session_serialize(void)
 		i++;
 	}
 	sh->header_ser->count = sh->count;
+	up_write(&sh->rwsem);
 
 	return 0;
 
@@ -602,6 +621,8 @@ int luo_session_serialize(void)
 		luo_session_unfreeze_one(session, &sh->ser[i]);
 		memset(sh->ser[i].name, 0, sizeof(sh->ser[i].name));
 	}
+	up_write(&sh->rwsem);
+	up_write(&luo_session_serialize_rwsem);
 
 	return err;
 }
-- 
2.53.0