Re: [PATCH v2] md/raid5: fix atomicity violation in raid5_cache_count

[Gui-Dong Han]

Dear All:

I hope this email finds you well. I hope you haven't missed my previous 
email, as I understand that everyone has a busy schedule. I just wanted 
to follow up on my previous message sent.
I understand that you may be occupied with other tasks or priorities. 
However, I would greatly appreciate it if you could spare a few moments 
to check the patch in my previous email. Your prompt response would be 
highly valuable to me.
Thank you for your attention to this matter, and I look forward to 
hearing from you soon.

Thanks,
Han

On 22/12/2023 下午12:52, Gui-Dong Han wrote:
> In raid5_cache_count():
> 	if (conf->max_nr_stripes < conf->min_nr_stripes)
> 		return 0;
> 	return conf->max_nr_stripes - conf->min_nr_stripes;
> The current check is ineffective, as the values could change immediately
> after being checked.
>
> In raid5_set_cache_size():
> 	...
> 	conf->min_nr_stripes = size;
> 	...
> 	while (size > conf->max_nr_stripes)
> 		conf->min_nr_stripes = conf->max_nr_stripes;
> 	...
>
> Due to intermediate value updates in raid5_set_cache_size(), concurrent
> execution of raid5_cache_count() and raid5_set_cache_size() may lead to
> inconsistent reads of conf->max_nr_stripes and conf->min_nr_stripes.
> The current checks are ineffective as values could change immediately
> after being checked, raising the risk of conf->min_nr_stripes exceeding
> conf->max_nr_stripes and potentially causing an integer overflow.
>
> This possible bug is found by an experimental static analysis tool
> developed by our team. This tool analyzes the locking APIs to extract
> function pairs that can be concurrently executed, and then analyzes the
> instructions in the paired functions to identify possible concurrency bugs
> including data races and atomicity violations. The above possible bug is
> reported when our tool analyzes the source code of Linux 6.2.
>
> To resolve this issue, it is suggested to introduce local variables
> 'min_stripes' and 'max_stripes' in raid5_cache_count() to ensure the
> values remain stable throughout the check. Adding locks in
> raid5_cache_count() fails to resolve atomicity violations, as
> raid5_set_cache_size() may hold intermediate values of
> conf->min_nr_stripes while unlocked. With this patch applied, our tool no
> longer reports the bug, with the kernel configuration allyesconfig for
> x86_64. Due to the lack of associated hardware, we cannot test the patch
> in runtime testing, and just verify it according to the code logic.
>
> Fixes: edbe83ab4c27e ("md/raid5: allow the stripe_cache to grow and ...")
> Reported-by: BassCheck <bass@xxxxxxxxxxx>
> Signed-off-by: Gui-Dong Han <2045gemini@xxxxxxxxx>
>
> ---
> v2:
> * In this patch v2, we've updated to use READ_ONCE() instead of direct
> reads for accessing max_nr_stripes and min_nr_stripes, since read and
> write can concurrent.
>    Thank Yu Kuai for helpful advice.
> ---
>   drivers/md/raid5.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 8497880135ee..9037e46de0e2 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -7391,10 +7391,12 @@ static unsigned long raid5_cache_count(struct shrinker *shrink,
>   {
>   	struct r5conf *conf = shrink->private_data;
>   
> -	if (conf->max_nr_stripes < conf->min_nr_stripes)
> +	int max_stripes = READ_ONCE(conf->max_nr_stripes);
> +	int min_stripes = READ_ONCE(conf->min_nr_stripes);
> +	if (max_stripes < min_stripes)
>   		/* unlikely, but not impossible */
>   		return 0;
> -	return conf->max_nr_stripes - conf->min_nr_stripes;
> +	return max_stripes - min_stripes;
>   }
>   
>   static struct r5conf *setup_conf(struct mddev *mddev)