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

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Message-ID: <YIg34SBa78u3KMNH@kroah.com>
Date:   Tue, 27 Apr 2021 18:12:17 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Peter Rosin <peda@...ntia.se>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 167/190] Revert "gdrom: fix a memory leak bug"

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 :)

thanks,

greg k-h