linux-kernel - Re: [PATCH v2] zram: fix possible use after free in zcomp

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Date:	Tue, 8 Sep 2015 10:07:18 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Luis Henriques <luis.henriques@...onical.com>
Cc:	Nitin Gupta <ngupta@...are.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] zram: fix possible use after free in zcomp_create()

Hello,

First of all, Thanks for catching a bug and review, Guys.
Below there are just some cleanup.
If you guys think it's better, please respin.

On Mon, Sep 07, 2015 at 03:13:10PM +0100, Luis Henriques wrote:
> zcomp_create() verifies the success of zcomp_strm_{multi,siggle}_create()
> through comp->stream, which can potentially be pointing to memory that was
> freed if these functions returned an error.
> 
> Fixes: beca3ec71fe5 ("zram: add multi stream functionality")
> Cc: stable@...r.kernel.org
> Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
> ---
> Changes since v1:
>  * Check zcomp_strm_{multi,siggle}_create() return code instead
>    comp->stream (suggested by Sergey)
> 
> drivers/block/zram/zcomp.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
> index 965d1afb0eaa..8d2cdfd307db 100644
> --- a/drivers/block/zram/zcomp.c
> +++ b/drivers/block/zram/zcomp.c
> @@ -336,6 +336,7 @@ struct zcomp *zcomp_create(const char *compress, int max_strm)
>  {
>  	struct zcomp *comp;
>  	struct zcomp_backend *backend;
> +	int ret;

For the clarification, I want to call it as 'error' instead of ret.

>  
>  	backend = find_backend(compress);
>  	if (!backend)
> @@ -347,10 +348,10 @@ struct zcomp *zcomp_create(const char *compress, int max_strm)
>  
>  	comp->backend = backend;
>  	if (max_strm > 1)
> -		zcomp_strm_multi_create(comp, max_strm);
> +		ret = zcomp_strm_multi_create(comp, max_strm);
>  	else
> -		zcomp_strm_single_create(comp);
> -	if (!comp->stream) {
> +		ret = zcomp_strm_single_create(comp);
> +	if (ret) {
>  		kfree(comp);
>  		return ERR_PTR(-ENOMEM);
>  	}

And we could return ERR_PTR(error) rather than fixed -ENOMEM to propagate
other errors potentially could be happen in future(ex, crypto support).
Of course, we should change description of the function about error return.

Thanks.
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