Re: [PATCH v3] kernel/module: Fix mem leak in module_add_modinfo_attrs

[Jessica Yu <jeyu@kernel.org>]

+++ Miroslav Benes [04/06/19 12:46 +0200]:
>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@huawei.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).
>
>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.

No, you are right in that it is a bit clumsy and and the sysfs error
path handling is asymmetrical. I think it could be cleaned up a bit.

IMO, I think the attr->free() calls should either be (1) removed from
module_remove_modinfo_attrs() as free_modinfo() takes care of that,
otherwise we could potentially call attr->free() twice (once in the
internal error handling of mod_sysfs_setup() and once again in the
free_modinfo: label in load_module()) or option (2) would be to merge
the attr->setup() calls into module_add_modinfo_attrs() so that it is
symmetrical to module_remove_modinfo_attrs(). I'm leaning towards
option 2 but have not carefully checked yet if moving the
attr->setup() calls into module_add_modinfo_attrs() would break
anything. In any case I will prepare some cleanup patches for this.

Thanks!

Jessica