linux-kernel - Re: [PATCH] block: Check blkg_to_lat return value to avoid NULL dereference

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Message-ID: <00f8157d-8c73-4aab-874c-77d0f5443365@wdc.com>
Date: Wed, 12 Feb 2025 09:02:48 +0000
From: Johannes Thumshirn <Johannes.Thumshirn@....com>
To: Wentao Liang <vulab@...as.ac.cn>, "tj@...nel.org" <tj@...nel.org>,
	"josef@...icpanda.com" <josef@...icpanda.com>, "axboe@...nel.dk"
	<axboe@...nel.dk>
CC: "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] block: Check blkg_to_lat return value to avoid NULL
 dereference

On 12.02.25 09:32, Wentao Liang wrote:
> The function blkg_to_lat() may return NULL if the blkg is not associated
> with an iolatency group. In iolatency_set_min_lat_nsec() and
> iolatency_pd_init(), the return values are not checked, leading to
> potential NULL pointer dereferences.
> 
> This patch adds checks for the return values of blkg_to_lat and let it
> returns early if it is NULL, preventing the NULL pointer dereference.
> 
> Fixes: d70675121546 ("block: introduce blk-iolatency io controller")
> Cc: stable@...r.kernel.org # 4.19+
> Signed-off-by: Wentao Liang <vulab@...as.ac.cn>
> ---
>   block/blk-iolatency.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
> index ebb522788d97..398f0a1747c4 100644
> --- a/block/blk-iolatency.c
> +++ b/block/blk-iolatency.c
> @@ -787,6 +787,8 @@ static int blk_iolatency_init(struct gendisk *disk)
>   static void iolatency_set_min_lat_nsec(struct blkcg_gq *blkg, u64 val)
>   {
>   	struct iolatency_grp *iolat = blkg_to_lat(blkg);
> +	if (!iolat)
> +		return;
>   	struct blk_iolatency *blkiolat = iolat->blkiolat;
>   	u64 oldval = iolat->min_lat_nsec;
>   

Uh that looks horrible. I haven't checked the surrounding code but 
please please at least make it

static void iolatency_set_min_lat_nsec(struct blkcg_gq *blkg, u64 val)
{
	struct iolatency_grp *iolat = blkg_to_lat(blkg);
	struct blk_iolatency *blkiolat;
	u64 oldval;

	if (!iolat)
		return;

	blkiolat = iolat->blkiolat;
	oldval = iolat->min_lat_nsec;



> @@ -1013,6 +1015,8 @@ static void iolatency_pd_init(struct blkg_policy_data *pd)
>   	 */
>   	if (blkg->parent && blkg_to_pd(blkg->parent, &blkcg_policy_iolatency)) {
>   		struct iolatency_grp *parent = blkg_to_lat(blkg->parent);
> +		if (!parent)
> +			return;
>   		atomic_set(&iolat->scale_cookie,
>   			   atomic_read(&parent->child_lat.scale_cookie));
>   	} else {