Re: [RFC PATCH] mm/damon/sysfs-schemes: hold damon_sysfs_lock in target_value_store()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] mm/damon/sysfs-schemes: uninitialized stack read in target_value_store()
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commit f43fea0015fcb1917385f72d150c59603a9ae43c
Author: SJ Park <sj@kernel.org>

mm/damon/sysfs-schemes: hold damon_sysfs_lock in target_value_store()

This commit holds damon_sysfs_lock in target_value_store() while updating
goal->target_value to prevent a time-of-check-to-time-of-use race with
damon_sysfs_add_quota_score() that could result in a divide-by-zero.

> diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
> index 8f083611741fd..55298a17b653a 100644
> --- a/mm/damon/sysfs-schemes.c
> +++ b/mm/damon/sysfs-schemes.c
> @@ -1355,7 +1355,13 @@ static ssize_t target_value_store(struct kobject *kobj,
>  {
>  	struct damos_sysfs_quota_goal *goal = container_of(kobj, struct
>  			damos_sysfs_quota_goal, kobj);
> -	int err = kstrtoul(buf, 0, &goal->target_value);
> +	unsigned long target_value;
> +	int err = kstrtoul(buf, 0, &target_value);
> +
> +	if (!mutex_trylock(&damon_sysfs_lock))
> +		return -EBUSY;
> +	goal->target_value = target_value;
> +	mutex_unlock(&damon_sysfs_lock);
>  
>  	return err ? err : count;
>  }

[Severity: High]
What happens in target_value_store() if kstrtoul() fails (for example,
returning -EINVAL or -ERANGE on malformed input) while damon_sysfs_lock is
not contended?

Since kstrtoul() leaves target_value untouched on error and err is not
checked before acquiring the lock, wouldn't this overwrite
goal->target_value with an uninitialized stack value (which could also be
read back via target_value_show())? Should err be checked and returned
immediately after kstrtoul()?

[]

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Sashiko AI review · https://sashiko.dev/#/patchset/20261020144726.88978-1-sj@kernel.org?part=1