linux-kernel - Re: [PATCH 167/190] Revert "gdrom: fix a memory leak bug"

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Date:   Tue, 27 Apr 2021 11:01:50 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Peter Rosin <peda@...ntia.se>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 167/190] Revert "gdrom: fix a memory leak bug"

On 4/27/21 10:12 AM, Greg KH wrote:
> On Tue, Apr 27, 2021 at 08:39:15AM -0600, Jens Axboe wrote:
>> On 4/27/21 8:03 AM, Peter Rosin wrote:
>>> On 2021-04-27 15:01, Greg KH wrote:
>>>> On Fri, Apr 23, 2021 at 08:20:30AM -0600, Jens Axboe wrote:
>>>>> On 4/22/21 3:29 PM, Peter Rosin wrote:
>>>>>>> This reverts commit 093c48213ee37c3c3ff1cf5ac1aa2a9d8bc66017.
>>>>>>
>>>>>> The reverted patch looks fishy.
>>>>>>
>>>>>> gc.cd_info is kzalloc:ed on probe. In case probe fails after this allocation, the
>>>>>> memory is kfree:d but the variable is NOT zeroed out.
>>>>>>
>>>>>> AFAICT, the above leads to a double-free on exit by the added line.
>>>>>>
>>>>>> I believe gd.cd_info should be kfree:d on remove instead.
>>>>>>
>>>>>> However, might not gc.toc also be kfree:d twice for similar reasons?
>>>>>>
>>>>>> I could easily be mistaken.
>>>>>
>>>>> >From taking a quick look the other day, that's my conclusion too. I
>>>>> don't think the patch is correct, but I don't think the surrounding code
>>>>> is correct right now either.
>>>>
>>>> Thanks for the review from both of you, I'll keep this commit in the
>>>> tree.
>>> Err, which commit is "this" and what tree are you keeping it in? I
>>> think you mean that you are keeping the revert in your tree with
>>> reverts, and not that you mean that we should keep the original
>>> commit in Linus' tree.
>>>
>>> In any case, I'd think that the original memory leak is somewhat
>>> better than the introduced double-free and therefore the revert
>>> should be done.
>>
>> It should probably look like the below, though I doubt it matters
>> since only one device is supported anyway. As long as the free
>> happens post unregister, it likely won't make a difference. But
>> it is cleaner and easier to verify, and should double device support
>> ever be introduced, the existing code is buggy.
>>
>> But given that, I don't think we should keep the revert patch.
>>
>> diff --git a/drivers/cdrom/gdrom.c b/drivers/cdrom/gdrom.c
>> index 9874fc1c815b..02d369881165 100644
>> --- a/drivers/cdrom/gdrom.c
>> +++ b/drivers/cdrom/gdrom.c
>> @@ -831,6 +831,8 @@ static int remove_gdrom(struct platform_device *devptr)
>>  	if (gdrom_major)
>>  		unregister_blkdev(gdrom_major, GDROM_DEV_NAME);
>>  	unregister_cdrom(gd.cd_info);
>> +	kfree(gd.toc);
>> +	kfree(gd.cd_info);
>>  
>>  	return 0;
>>  }
>> @@ -862,8 +864,6 @@ static void __exit exit_gdrom(void)
>>  {
>>  	platform_device_unregister(pd);
>>  	platform_driver_unregister(&gdrom_driver);
>> -	kfree(gd.toc);
>> -	kfree(gd.cd_info);
>>  }
>>  
>>  module_init(init_gdrom);
>>
>> -- 
>> Jens Axboe
>>
> 
> I'll add this fix to the tree after the revert, and give you the credit
> for the fix :)

Sounds good, thanks Greg.

-- 
Jens Axboe