[PATCH 2/2] ext4: Fix potential uas-after-free about sbi->s_mmp_tsk when kmmpd kthread exit before set sbi->s_mmp_tsk

[Ye Bin]

Now sbi->s_mmp_tsk is created with kthread_run, then kmmpd maybe
already running and even exit as exception. Even though we set
sbi->s_mmp_tsk with NULL before kmmpd kthread exit, but
"sbi->s_mmp_tsk=kthread_run(XX)" may set after set with NULL.
   mount                     kmmpd
     |                         |
     |-call kthread_run        |
     |                         |-kmmpd runing
     |                         |-kmmpd exit sbi->s_mmp_tsk=NULL
     |                         |
     |-kthread_run return      |
     | and set sbi->s_mmp_tsk  |
     |                         |
     |-then we get wild ptr"sbi->s_mmp_tsk" and later trigger UAF

This patch is base on previous "ext4: Fix use-after-free about sbi->s_mmp_tsk".
Previous patch ensure kmmpd kthread exit by itself will set sbi->s_mmp_tsk with
NULL. We can create kthread first, and then wakeup kmmpd kthread later.

Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
---
 fs/ext4/mmp.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index fc18a8c205c7..6ec1ea182cc0 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -394,16 +394,18 @@ int ext4_multi_mount_protect(struct super_block *sb,
 	/*
 	 * Start a kernel thread to update the MMP block periodically.
 	 */
-	EXT4_SB(sb)->s_mmp_tsk = kthread_run(kmmpd, sb, "kmmpd-%.*s",
-					     (int)sizeof(mmp->mmp_bdevname),
-					     bdevname(bh->b_bdev,
-						      mmp->mmp_bdevname));
+	EXT4_SB(sb)->s_mmp_tsk = kthread_create(kmmpd, sb, "kmmpd-%.*s",
+						(int)sizeof(mmp->mmp_bdevname),
+						bdevname(bh->b_bdev,
+							 mmp->mmp_bdevname));
+
 	if (IS_ERR(EXT4_SB(sb)->s_mmp_tsk)) {
 		EXT4_SB(sb)->s_mmp_tsk = NULL;
 		ext4_warning(sb, "Unable to create kmmpd thread for %s.",
 			     sb->s_id);
 		goto failed;
 	}
+	wake_up_process(EXT4_SB(sb)->s_mmp_tsk);
 
 	return 0;
 
-- 
2.31.1