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From:   Andreas Dilger <adilger@dilger.ca>
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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
Date:   Mon, 5 Jul 2021 18:44:20 -0600
In-Reply-To: <20210629143603.2166962-3-yebin10@huawei.com>
Cc:     Theodore Ts'o <tytso@mit.edu>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jan Kara <jack@suse.cz>
To:     Ye Bin <yebin10@huawei.com>
References: <20210629143603.2166962-1-yebin10@huawei.com>
 <20210629143603.2166962-3-yebin10@huawei.com>
Precedence: bulk


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On Jun 29, 2021, at 8:36 AM, Ye Bin <yebin10@huawei.com> wrote:
>=20
> 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=3Dkthread_run(XX)" may set after set with NULL.
>   mount                     kmmpd
>     |                         |
>     |-call kthread_run        |
>     |                         |-kmmpd runing
>     |                         |-kmmpd exit sbi->s_mmp_tsk=3DNULL
>     |                         |
>     |-kthread_run return      |
>     | and set sbi->s_mmp_tsk  |
>     |                         |
>     |-then we get wild ptr"sbi->s_mmp_tsk" and later trigger UAF
>=20
> 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.
>=20
> 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(-)
>=20
> 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 =3D kthread_run(kmmpd, sb, "kmmpd-%.*s",
> -					     =
(int)sizeof(mmp->mmp_bdevname),
> -					     bdevname(bh->b_bdev,
> -						      =
mmp->mmp_bdevname));
> +	EXT4_SB(sb)->s_mmp_tsk =3D 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 =3D 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);
>=20
> 	return 0;
>=20
> --
> 2.31.1
>=20


Cheers, Andreas


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