linux-kernel - Re: [PATCH] cfq: fix NULL pointer dereference in cfq_choose

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Message-ID: <CAFLxGvyt_-WFLe_WJGGz4oztSAiYoKk1+BD9_OsK4toSjP4V5Q@mail.gmail.com>
Date:	Sun, 4 Mar 2012 13:29:19 +0100
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	Namjae Jeon <linkinjeon@...il.com>
Cc:	axboe@...nel.dk, tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cfq: fix NULL pointer dereference in cfq_choose_cfqg.

On Sun, Mar 4, 2012 at 1:10 PM, Namjae Jeon <linkinjeon@...il.com> wrote:
> I found null pointer dereferencing possibility while reviewing.
> If cfq_get_next_cfqg return null by RB_EMPTY_ROOT, null pointer dereferencing error can occur.
>
> Signed-off-by: Namjae Jeon <linkinjeon@...il.com>
> ---
>  block/cfq-iosched.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index d0ba505..d924272 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -2306,7 +2306,7 @@ static void cfq_choose_cfqg(struct cfq_data *cfqd)
>        cfqd->serving_group = cfqg;
>
>        /* Restore the workload type data */
> -       if (cfqg->saved_workload_slice) {
> +       if (!cfqg && cfqg->saved_workload_slice) {
>                cfqd->workload_expires = jiffies + cfqg->saved_workload_slice;
>                cfqd->serving_type = cfqg->saved_workload;
>                cfqd->serving_prio = cfqg->saved_serving_prio;

I don't think that this is a correct fix.

1. Can it really happen that cfq_get_next_cfqg() returns NULL at this point?

2. Adding a if(!cfgg ...) does not solve the problem, it ignores it.
After the if block we call choose_service_tree().
I'm sure choose_service_tree() it not happy when we pass a NULL pointer to it...

-- 
Thanks,
//richard
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