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

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Date:   Mon, 5 Jul 2021 12:52:41 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ye Bin <yebin10@...wei.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        jack@...e.cz
Subject: Re: [PATCH 2/2] ext4: Fix potential uas-after-free about
 sbi->s_mmp_tsk when kmmpd kthread exit before set sbi->s_mmp_tsk

On Tue 29-06-21 22:36:03, Ye Bin wrote:
> 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@...wei.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR