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

[Andreas Dilger <adilger@dilger.ca>]

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On Jun 29, 2021, at 8:36 AM, Ye Bin <yebin10@huawei.com> 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@huawei.com>

Please note minor typo in the patch subject, should be "use-after-free".

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

> ---
> 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
> 


Cheers, Andreas






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