Re: [PATCH v3] md/raid5: fix atomicity violation in raid5_cache_count

[Yu Kuai]

在 2024/01/11 16:27, Gui-Dong Han 写道:
> 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.
This patch LGTM, just one nit below.

> Fixes: edbe83ab4c27 ("md/raid5: allow the stripe_cache to grow and ...")
> 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.
> ---
> v3:
> * In this patch v3, we've updated to use WRITE_ONCE() in
> raid5_set_cache_size(), grow_one_stripe() and drop_one_stripe(), in order
> to pair READ_ONCE() with WRITE_ONCE().
>    Thank Yu Kuai for helpful advice.
> ---
>   drivers/md/raid5.c | 14 ++++++++------
>   1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 26e1e8a5e941..ece2b2094dc0 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -2422,7 +2422,7 @@ static int grow_one_stripe(struct r5conf *conf, gfp_t gfp)
>   	atomic_inc(&conf->active_stripes);
>   
>   	raid5_release_stripe(sh);
> -	conf->max_nr_stripes++;
> +	WRITE_ONCE(conf->max_nr_stripes, conf->max_nr_stripes + 1);
>   	return 1;
>   }
>   
> @@ -2717,7 +2717,7 @@ static int drop_one_stripe(struct r5conf *conf)
>   	shrink_buffers(sh);
>   	free_stripe(conf->slab_cache, sh);
>   	atomic_dec(&conf->active_stripes);
> -	conf->max_nr_stripes--;
> +	WRITE_ONCE(conf->max_nr_stripes, conf->max_nr_stripes - 1);
>   	return 1;
>   }
>   
> @@ -6878,7 +6878,7 @@ raid5_set_cache_size(struct mddev *mddev, int size)
>   	if (size <= 16 || size > 32768)
>   		return -EINVAL;
>   
> -	conf->min_nr_stripes = size;
> +	WRITE_ONCE(conf->min_nr_stripes, size);
>   	mutex_lock(&conf->cache_size_mutex);
>   	while (size < conf->max_nr_stripes &&
>   	       drop_one_stripe(conf))
> @@ -6890,7 +6890,7 @@ raid5_set_cache_size(struct mddev *mddev, int size)
>   	mutex_lock(&conf->cache_size_mutex);
>   	while (size > conf->max_nr_stripes)
>   		if (!grow_one_stripe(conf, GFP_KERNEL)) {
> -			conf->min_nr_stripes = conf->max_nr_stripes;
> +			WRITE_ONCE(conf->min_nr_stripes, conf->max_nr_stripes);
>   			result = -ENOMEM;
>   			break;
>   		}
> @@ -7449,10 +7449,12 @@ static unsigned long raid5_cache_count(struct shrinker *shrink,
>   {
>   	struct r5conf *conf = shrink->private_data;
>   
You can remove this empty line now.
> -	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);

And please keep an emptly line between declaration. (do you check this
patch with ./scripts/checkpatch.pl?)

Thanks,
Kuai
> +	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)