Re: [PATCH] eventfs: fix a null pointer access in eventfs_iterate

[Chi Zhiling <chizhiling@163.com>]

On 2024/9/4 03:51, Steven Rostedt wrote:
> On Thu, 29 Aug 2024 11:24:36 +0800
> Chi Zhiling <chizhiling@163.com> wrote:
>
>> From: Chi Zhiling <chizhiling@kylinos.cn>
>>
>> We found a null pointer accessing in tracefs[1], the reason is that
>> the variable 'ei_child' is set to LIST_POISON1, that means the list
>> was removed in eventfs_remove_rec. so when access the ei_child->is_freed,
>> the panic triggered.
>>
>> the linked list is protected by eventfs_mutex in eventfs_remove_rec,
> Only writes of the link list is protected by the mutex. Reads are not.
>
>> so when we access the list of ei_child in eventfs_iterate, we also need
>> a mutex_lock in there to avoid eventfs_remove_rec modify the list.
> Yes you hit a bug, but no this is *not* the solution!
>
>
>> Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
>> ---
>>   fs/tracefs/event_inode.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
>> index 01e99e98457d..4895ed07376b 100644
>> --- a/fs/tracefs/event_inode.c
>> +++ b/fs/tracefs/event_inode.c
>> @@ -642,6 +642,7 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
>>   	/* Subtract the skipped entries above */
>>   	c -= min((unsigned int)c, (unsigned int)ei->nr_entries);
>>   
>> +	mutex_unlock(&eventfs_mutex);
> This list is protected by SRCU (hence the name of the iterator), if you
> need to add a mutex around it, something else is broken.
>
>>   	list_for_each_entry_srcu(ei_child, &ei->children, list,
>>   				 srcu_read_lock_held(&eventfs_srcu)) {
>>   
>> @@ -659,9 +660,12 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
>>   
>>   		ino = eventfs_dir_ino(ei_child);
>>   
>> -		if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR))
>> +		if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR)) {
>> +			mutex_unlock(&eventfs_mutex);
>>   			goto out_dec;
>> +		}
>>   	}
>> +	mutex_unlock(&eventfs_mutex);
>>   	ret = 1;
>>    out:
>>   	srcu_read_unlock(&eventfs_srcu, idx);
> The real fix is:
>
> diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
> index 01e99e98457d..8705c77a9e75 100644
> --- a/fs/tracefs/event_inode.c
> +++ b/fs/tracefs/event_inode.c
> @@ -862,7 +862,7 @@ static void eventfs_remove_rec(struct eventfs_inode *ei, int level)
>   	list_for_each_entry(ei_child, &ei->children, list)
>   		eventfs_remove_rec(ei_child, level + 1);
>   
> -	list_del(&ei->list);
> +	list_del_rcu(&ei->list);
>   	free_ei(ei);
>   }
>   
> Can you test that and let me know if it fixes your issue. I'll just go
> ahead and apply it as it is an obvious bug.
>
Okay, I've tested it and I haven't seen any errors.