[PATCH] cfq: fix NULL pointer dereference in cfq_choose_cfqg.

[PATCH] cfq: fix NULL pointer dereference in cfq_choose_cfqg.

[Namjae Jeon]

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@gmail.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;
-- 
1.7.5.4

Re: [PATCH] cfq: fix NULL pointer dereference in cfq_choose_cfqg.

[richard -rw- weinberger]

On Sun, Mar 4, 2012 at 1:10 PM, Namjae Jeon <linkinjeon@gmail.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@gmail.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

Re: [PATCH] cfq: fix NULL pointer dereference in cfq_choose_cfqg.

[Namjae Jeon]

Hi. Richard.

You're right. need to add exception code more. I will look for more.

Thanks.

2012/3/4 richard -rw- weinberger <richard.weinberger@gmail.com>:
> On Sun, Mar 4, 2012 at 1:10 PM, Namjae Jeon <linkinjeon@gmail.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@gmail.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