Forwarded: Re: [syz] KASAN: use-after-free Read in jfs_lazycommit

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Subject: Re: [syz] KASAN: use-after-free Read in jfs_lazycommit
Author: tristmd@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

>From a6c188149cf9056a8692aa45dfd0d429bd07d09f Mon Sep 17 00:00:00 2001
From: Tristan Madani <tristan@talencesecurity.com>
Date: Thu, 30 Apr 2026 23:12:58 +0000
Subject: [PATCH] jfs: drain lazy commit queue during unmount to prevent
 use-after-free

The jfsCommit kernel thread processes committed transactions from
TxAnchor.unlock_queue via jfs_lazycommit().  During filesystem
unmount, jfs_umount() calls jfs_flush_journal(log, 2) which waits
for the log commit queue (log->cqueue) to drain.  However, after
log I/O completes, lazy transactions are moved to
TxAnchor.unlock_queue for asynchronous processing by jfsCommit.

If jfs_umount() proceeds to free the jfs_log (via lmLogClose) or
jfs_sb_info (via kfree in jfs_put_super) while entries referencing
this superblock remain on unlock_queue, the jfsCommit thread will
access freed memory when it later processes these entries:

- jfs_lazycommit reads sbi->commit_state (UAF of jfs_sb_info)
- txLazyCommit accesses JFS_SBI(tblk->sb)->log and takes
  log->gclock (UAF of jfs_log)

Add txLazyDrain() which waits for all entries in
TxAnchor.unlock_queue belonging to the unmounting superblock to be
processed, and also waits for any in-flight txLazyCommit
(IN_LAZYCOMMIT) for this superblock to complete.  Call it from both
jfs_umount() and jfs_umount_rw() after jfs_flush_journal().

Reported-by: syzbot+c244f4a09ca85dd2ebc1@syzkaller.appspotmail.com
Reported-by: syzbot+885a4f3281b8d99c48d8@syzkaller.appspotmail.com
Fixes: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
 fs/jfs/jfs_txnmgr.c | 34 ++++++++++++++++++++++++++++++++++
 fs/jfs/jfs_txnmgr.h |  1 +
 fs/jfs/jfs_umount.c |  8 ++++++++
 3 files changed, 43 insertions(+)

diff --git a/fs/jfs/jfs_txnmgr.c b/fs/jfs/jfs_txnmgr.c
index 083dbbb0c3268..e3b2cee416c19 100644
--- a/fs/jfs/jfs_txnmgr.c
+++ b/fs/jfs/jfs_txnmgr.c
@@ -2791,6 +2791,40 @@ void txLazyUnlock(struct tblock * tblk)
 	LAZY_UNLOCK(flags);
 }
 
+
+/*
+ * txLazyDrain
+ *
+ * Wait for all pending lazy commit entries for this superblock
+ * to be processed by the jfsCommit thread.  Must be called
+ * before freeing per-filesystem structures during unmount.
+ */
+void txLazyDrain(struct super_block *sb)
+{
+	struct jfs_sb_info *sbi = JFS_SBI(sb);
+	struct tblock *tblk;
+	unsigned long flags;
+	bool found;
+
+	do {
+		found = false;
+		LAZY_LOCK(flags);
+		list_for_each_entry(tblk, &TxAnchor.unlock_queue, cqueue) {
+			if (tblk->sb == sb) {
+				found = true;
+				break;
+			}
+		}
+		if (!found && (sbi->commit_state & IN_LAZYCOMMIT))
+			found = true;
+		LAZY_UNLOCK(flags);
+
+		if (found) {
+			wake_up(&jfs_commit_thread_wait);
+			schedule_timeout_uninterruptible(1);
+		}
+	} while (found);
+}
 static void LogSyncRelease(struct metapage * mp)
 {
 	struct jfs_log *log = mp->log;
diff --git a/fs/jfs/jfs_txnmgr.h b/fs/jfs/jfs_txnmgr.h
index ba71eb5ced567..fbbaed26c52bd 100644
--- a/fs/jfs/jfs_txnmgr.h
+++ b/fs/jfs/jfs_txnmgr.h
@@ -291,6 +291,7 @@ extern void txFreelock(struct inode *);
 extern int lmLog(struct jfs_log *, struct tblock *, struct lrd *,
 		 struct tlock *);
 extern void txQuiesce(struct super_block *);
+extern void txLazyDrain(struct super_block *);
 extern void txResume(struct super_block *);
 extern void txLazyUnlock(struct tblock *);
 extern int jfs_lazycommit(void *);
diff --git a/fs/jfs/jfs_umount.c b/fs/jfs/jfs_umount.c
index 18569f1eaabdb..657707361be2a 100644
--- a/fs/jfs/jfs_umount.c
+++ b/fs/jfs/jfs_umount.c
@@ -58,6 +58,13 @@ int jfs_umount(struct super_block *sb)
 		 */
 		jfs_flush_journal(log, 2);
 
+	/*
+	 * Drain any pending lazy commit entries for this filesystem so
+	 * the jfsCommit thread does not access freed structures.
+	 */
+	if (log)
+		txLazyDrain(sb);
+
 	/*
 	 * Hold log lock so write_special_inodes (lmLogSync) cannot see
 	 * this sbi with a NULL inode pointer while iterating log->sb_list.
@@ -142,6 +149,7 @@ int jfs_umount_rw(struct super_block *sb)
 	 * remove file system from log active file system list.
 	 */
 	jfs_flush_journal(log, 2);
+	txLazyDrain(sb);
 
 	/*
 	 * Make sure all metadata makes it to disk
-- 
2.47.3