[PATCH 6.1 060/183] [PATCH AUTOSEL 4.19 10/11] FS: JFS: Fix null-ptr-deref Read in txBegin

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

[ Upstream commit 47cfdc338d674d38f4b2f22b7612cc6a2763ba27 ]

 Syzkaller reported an issue where txBegin may be called
 on a superblock in a read-only mounted filesystem which leads
 to NULL pointer deref. This could be solved by checking if
 the filesystem is read-only before calling txBegin, and returning
 with appropiate error code.

Reported-By: syzbot+f1faa20eec55e0c8644c@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://syzkaller.appspot.com/bug?id=be7e52c50c5182cc09a09ea6fc456446b2039de3

Signed-off-by: Immad Mir <mirimmad17@xxxxxxxxx>
Signed-off-by: Dave Kleikamp <dave.kleikamp@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/jfs/namei.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/fs/jfs/namei.c
+++ b/fs/jfs/namei.c
@@ -799,6 +799,11 @@ static int jfs_link(struct dentry *old_d
 	if (rc)
 		goto out;
 
+	if (isReadOnly(ip)) {
+		jfs_error(ip->i_sb, "read-only filesystem\n");
+		return -EROFS;
+	}
+
 	tid = txBegin(ip->i_sb, 0);
 
 	mutex_lock_nested(&JFS_IP(dir)->commit_mutex, COMMIT_MUTEX_PARENT);