Re: [PATCH] mtd: ubi: Fix UBI device reference leak in ubi_detach_mtd_dev()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] mtd: ubi: double put of ubi_device in ubi_detach_mtd_dev()
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commit dc1e1560c6a92e217dc3827eb13a6c41acadcafb
Author: Wentao Liang <vulab@xxxxxxxxxxx>

mtd: ubi: Fix UBI device reference leak in ubi_detach_mtd_dev()

ubi_detach_mtd_dev() takes a reference on the UBI device with ubi_get_device()
and attempts to drop it. This patch aims to fix a reference leak when returning
-EBUSY.

> diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
> index a901f8edfa41d..866638c006494 100644
> --- a/drivers/mtd/ubi/build.c
> +++ b/drivers/mtd/ubi/build.c
> @@ -1073,6 +1073,7 @@ int ubi_detach_mtd_dev(int ubi_num, int anyway)
>  	if (ubi->ref_count) {
>  		if (!anyway) {
>  			spin_unlock(&ubi_devices_lock);
> +			put_device(&ubi->dev);

[Severity: High]
Does this code double-put the ubi_device on the error path?

Looking at ubi_detach_mtd_dev() in drivers/mtd/ubi/build.c, put_device() is
already called right before the ref_count check:

	spin_lock(&ubi_devices_lock);
	put_device(&ubi->dev);
	ubi->ref_count -= 1;
	if (ubi->ref_count) {
		if (!anyway) {
			spin_unlock(&ubi_devices_lock);

Adding another put_device(&ubi->dev) here drops a kobject reference that the
function does not own, leading to premature freeing of struct ubi_device memory
via dev_release while it is still in use by other kernel components.

If a user with CAP_SYS_RESOURCE calls the UBI_IOCDET ioctl on the UBI control
device while the target UBI device is in use, ubi->ref_count is > 0, leading
ubi_detach_mtd_dev() to return -EBUSY and execute this path.

This appears to be a flawed backport intended for a newer kernel tree where
put_device() was actually missing from this block (due to commit 7e84c961b2eb).
In this baseline, the reference is already correctly dropped before the -EBUSY
check.

>  			return -EBUSY;
>  		}
>  		/* This may only happen if there is a bug */

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Sashiko AI review · https://sashiko.dev/#/patchset/20260917103102.2147038-1-vulab@xxxxxxxxxxx?part=1