linux-kernel - Re: [PATCH v2 4/4] fs: unicode: Add utf8 module and a unicode layer

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Date:   Fri, 19 Mar 2021 09:34:44 -0400
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     Shreeya Patel <shreeya.patel@...labora.com>
Cc:     jaegeuk@...nel.org, yuchao0@...wei.com, tytso@....edu,
        adilger.kernel@...ger.ca, drosen@...gle.com, ebiggers@...gle.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        kernel@...labora.com, andre.almeida@...labora.com
Subject: Re: [PATCH v2 4/4] fs: unicode: Add utf8 module and a unicode layer

Shreeya Patel <shreeya.patel@...labora.com> writes:

> On 19/03/21 1:27 am, Gabriel Krisman Bertazi wrote:

>> Maybe, the if leg should be:
>>
>> if (!utf8_ops || !try_module_get(utf8_ops->owner)
>>     return ERR_PTR(-ENODEV)
>>
>> But this is still racy, since you are not protecting utf8_ops before
>> acquiring the reference.  If you race with module removal here, a
>> NULL ptr dereference can still occur.  See below.
>
>
> If module is removed before reaching this step, then unicode_unregister
> function would make utf8_ops NULL. So the first condition of if will be true
> and it will return error so how can we have a NULL ptr dereference
> then?

Hi Shreeya,

As we discussed offline, it can happen if the module is deregistered
after checking utf8_ops and before doing the try_module_get.

>>>   }
>>> -EXPORT_SYMBOL(unicode_normalize);
>>> +EXPORT_SYMBOL(unicode_load);
>>>   -static int unicode_parse_version(const char *version, unsigned int
>>> *maj,
>>> -				 unsigned int *min, unsigned int *rev)
>>> +void unicode_unload(struct unicode_map *um)
>>>   {
>>> -	substring_t args[3];
>>> -	char version_string[12];
>>> -	static const struct match_token token[] = {
>>> -		{1, "%d.%d.%d"},
>>> -		{0, NULL}
>>> -	};
>>> -
>>> -	strscpy(version_string, version, sizeof(version_string));
>>> -
>>> -	if (match_token(version_string, token, args) != 1)
>>> -		return -EINVAL;
>>> +	if (utf8_ops)
>>> +		module_put(utf8_ops->owner);
>>>
>> How can we have a unicode_map to free if utf8_ops is NULL?  that seems
>> to be an invalid use of API, which suggests a bug elsewhere
>> in the kernel.  maybe this should read like this:
>>
>> void unicode_unload(struct unicode_map *um)
>> {
>>    if (WARN_ON(!utf8_ops))
>>      return;
>>
>>    module_put(utf8_ops->owner);
>>    kfree(um);
>> }
>
>
> The reason for adding the check if(utf8_ops) is that some of the filesystem
> calls the unicode_unload function even before calling the unicode_load
> function.
> if we try to decrement the reference without even having the
> reference. ( i.e. not loading the module )
> it would result in kernel panic.
> fs/ext4/super.c
> fs/f2fs/super.c
> Both the above files call the unicode_unload function if CONFIG_UNICODE
> is enabled.
> Not sure if this is an odd behavior or expected.

Those seem to be error paths, where the mount fails before we get a
chance to load the unicode map.  I suggest we fix the callers to avoid
calling the unicode API unnecessarily.

-- 
Gabriel Krisman Bertazi