linux-kernel - Re: [PATCH v3] kernel/module: Fix mem leak in module_add_modinfo

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Date:   Tue, 4 Jun 2019 21:54:44 +0800
From:   Yuehaibing <yuehaibing@...wei.com>
To:     Miroslav Benes <mbenes@...e.cz>
CC:     <jeyu@...nel.org>, <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] kernel/module: Fix mem leak in
 module_add_modinfo_attrs

On 2019/6/4 18:46, Miroslav Benes wrote:
> On Mon, 3 Jun 2019, YueHaibing wrote:
> 
>> In module_add_modinfo_attrs if sysfs_create_file
>> fails, we forget to free allocated modinfo_attrs
>> and roll back the sysfs files.
>>
>> Fixes: 03e88ae1b13d ("[PATCH] fix module sysfs files reference counting")
>> Signed-off-by: YueHaibing <yuehaibing@...wei.com>
>> ---
>> v3: reuse module_remove_modinfo_attrs
>> v2: free from '--i' instead of 'i--'
>> ---
>>  kernel/module.c | 21 ++++++++++++++++-----
>>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> I'm afraid it is not completely correct.
>  
>> diff --git a/kernel/module.c b/kernel/module.c
>> index 80c7c09..c6b8912 100644
>> --- a/kernel/module.c
>> +++ b/kernel/module.c
>> @@ -1697,6 +1697,8 @@ static int add_usage_links(struct module *mod)
>>  	return ret;
>>  }
>>  
>> +static void module_remove_modinfo_attrs(struct module *mod, int end);
>> +
>>  static int module_add_modinfo_attrs(struct module *mod)
>>  {
>>  	struct module_attribute *attr;
>> @@ -1711,24 +1713,33 @@ static int module_add_modinfo_attrs(struct module *mod)
>>  		return -ENOMEM;
>>  
>>  	temp_attr = mod->modinfo_attrs;
>> -	for (i = 0; (attr = modinfo_attrs[i]) && !error; i++) {
>> +	for (i = 0; (attr = modinfo_attrs[i]); i++) {
>>  		if (!attr->test || attr->test(mod)) {
>>  			memcpy(temp_attr, attr, sizeof(*temp_attr));
>>  			sysfs_attr_init(&temp_attr->attr);
>>  			error = sysfs_create_file(&mod->mkobj.kobj,
>>  					&temp_attr->attr);
>> +			if (error)
>> +				goto error_out;
> 
> sysfs_create_file() failed, so we need to clear all previously processed 
> attrs and not the current one.
> 
>>  			++temp_attr;
>>  		}
>>  	}
>> +
>> +	return 0;
>> +
>> +error_out:
>> +	module_remove_modinfo_attrs(mod, --i);
> 
> It says "call sysfs_remove_file() on all attrs ending with --i included 
> (all correctly processed attrs).
> 
>>  	return error;
>>  }
>>  
>> -static void module_remove_modinfo_attrs(struct module *mod)
>> +static void module_remove_modinfo_attrs(struct module *mod, int end)
>>  {
>>  	struct module_attribute *attr;
>>  	int i;
>>  
>>  	for (i = 0; (attr = &mod->modinfo_attrs[i]); i++) {
>> +		if (end >= 0 && i > end)
>> +			break;
> 
> If end == 0, you break the loop without calling sysfs_remove_file(), which 
> is a bug if you called module_remove_modinfo_attrs(mod, 0).

If end == 0 and i == 0, if statement is false, it won't break the loop.

At other places, I use end == -1, which means clean all and keeps the old behavior

-	module_remove_modinfo_attrs(mod);
+	module_remove_modinfo_attrs(mod, -1);


> 
> Calling module_remove_modinfo_attrs(mod, i); in module_add_modinfo_attrs() 
> under error_out label and changing the condition here to 
> 
> if (end >= 0 && i >= end)
> 	break;
> 
> should work as expected.
> 
> But let me ask another question and it might be more to Jessica. Why is 
> there even a call to attr->free(mod); (if it exists) in 
> module_remove_modinfo_attrs()? The same is in free_modinfo() (as opposed 
> to setup_modinfo() where attr->setup(mod) is called. Is it because 
> free_modinfo() is called only in load_module()'s error path, while 
> module_remove_modinfo_attrs() is called even in free_module() path?
> 
> kfree() checks for NULL pointer, so there is no bug, but it is certainly 
> not nice and it calls for cleanup. But I may be missing something.
> 
> Regards,
> Miroslav
> 
> .
>