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Date:   Wed, 7 Jul 2021 11:30:28 +0200
From:   Jan Kara <jack@...e.cz>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Ye Bin <yebin10@...wei.com>
Subject: Re: [PATCH -v4] ext4: fix possible UAF when remounting r/o a
 mmp-protected file system

On Tue 06-07-21 20:24:33, Theodore Ts'o wrote:
> After commit 618f003199c6 ("ext4: fix memory leak in
> ext4_fill_super"), after the file system is remounted read-only, there
> is a race where the kmmpd thread can exit, causing sbi->s_mmp_tsk to
> point at freed memory, which the call to ext4_stop_mmpd() can trip
> over.
> 
> Fix this by only allowing kmmpd() to exit when it is stopped via
> ext4_stop_mmpd().
> 
> Link: https://lore.kernel.org/r/YONtEGojq7LcXnuC@mit.edu
> Reported-by: Ye Bin <yebin10@...wei.com>
> Bug-Report-Link: <20210629143603.2166962-1-yebin10@...wei.com>
> Signed-off-by: Theodore Ts'o <tytso@....edu>

Looks good. Feel free to add:

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

								Honza

> ---
>  fs/ext4/mmp.c   | 31 +++++++++++++++----------------
>  fs/ext4/super.c |  6 +++++-
>  2 files changed, 20 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
> index 6cb598b549ca..bc364c119af6 100644
> --- a/fs/ext4/mmp.c
> +++ b/fs/ext4/mmp.c
> @@ -156,7 +156,12 @@ static int kmmpd(void *data)
>  	memcpy(mmp->mmp_nodename, init_utsname()->nodename,
>  	       sizeof(mmp->mmp_nodename));
>  
> -	while (!kthread_should_stop()) {
> +	while (!kthread_should_stop() && !sb_rdonly(sb)) {
> +		if (!ext4_has_feature_mmp(sb)) {
> +			ext4_warning(sb, "kmmpd being stopped since MMP feature"
> +				     " has been disabled.");
> +			goto wait_to_exit;
> +		}
>  		if (++seq > EXT4_MMP_SEQ_MAX)
>  			seq = 1;
>  
> @@ -177,16 +182,6 @@ static int kmmpd(void *data)
>  			failed_writes++;
>  		}
>  
> -		if (!(le32_to_cpu(es->s_feature_incompat) &
> -		    EXT4_FEATURE_INCOMPAT_MMP)) {
> -			ext4_warning(sb, "kmmpd being stopped since MMP feature"
> -				     " has been disabled.");
> -			goto exit_thread;
> -		}
> -
> -		if (sb_rdonly(sb))
> -			break;
> -
>  		diff = jiffies - last_update_time;
>  		if (diff < mmp_update_interval * HZ)
>  			schedule_timeout_interruptible(mmp_update_interval *
> @@ -207,7 +202,7 @@ static int kmmpd(void *data)
>  				ext4_error_err(sb, -retval,
>  					       "error reading MMP data: %d",
>  					       retval);
> -				goto exit_thread;
> +				goto wait_to_exit;
>  			}
>  
>  			mmp_check = (struct mmp_struct *)(bh_check->b_data);
> @@ -221,7 +216,7 @@ static int kmmpd(void *data)
>  				ext4_error_err(sb, EBUSY, "abort");
>  				put_bh(bh_check);
>  				retval = -EBUSY;
> -				goto exit_thread;
> +				goto wait_to_exit;
>  			}
>  			put_bh(bh_check);
>  		}
> @@ -244,7 +239,13 @@ static int kmmpd(void *data)
>  
>  	retval = write_mmp_block(sb, bh);
>  
> -exit_thread:
> +wait_to_exit:
> +	while (!kthread_should_stop()) {
> +		set_current_state(TASK_INTERRUPTIBLE);
> +		if (!kthread_should_stop())
> +			schedule();
> +	}
> +	set_current_state(TASK_RUNNING);
>  	return retval;
>  }
>  
> @@ -391,5 +392,3 @@ int ext4_multi_mount_protect(struct super_block *sb,
>  	brelse(bh);
>  	return 1;
>  }
> -
> -
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index cdbe71d935e8..b8ff0399e171 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -5993,7 +5993,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
>  				 */
>  				ext4_mark_recovery_complete(sb, es);
>  			}
> -			ext4_stop_mmpd(sbi);
>  		} else {
>  			/* Make sure we can mount this feature set readwrite */
>  			if (ext4_has_feature_readonly(sb) ||
> @@ -6107,6 +6106,9 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
>  	if (!test_opt(sb, BLOCK_VALIDITY) && sbi->s_system_blks)
>  		ext4_release_system_zone(sb);
>  
> +	if (!ext4_has_feature_mmp(sb) || sb_rdonly(sb))
> +		ext4_stop_mmpd(sbi);
> +
>  	/*
>  	 * Some options can be enabled by ext4 and/or by VFS mount flag
>  	 * either way we need to make sure it matches in both *flags and
> @@ -6140,6 +6142,8 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
>  	for (i = 0; i < EXT4_MAXQUOTAS; i++)
>  		kfree(to_free[i]);
>  #endif
> +	if (!ext4_has_feature_mmp(sb) || sb_rdonly(sb))
> +		ext4_stop_mmpd(sbi);
>  	kfree(orig_data);
>  	return err;
>  }
> -- 
> 2.31.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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