linux-kernel - kobject_init_and_add is easy to misuse

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Message-ID: <20200602121035.GL19604@bombadil.infradead.org>
Date:   Tue, 2 Jun 2020 05:10:35 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Wang Hai <wanghai38@...wei.com>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        khlebnikov@...dex-team.ru, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: kobject_init_and_add is easy to misuse

On Tue, Jun 02, 2020 at 07:50:33PM +0800, Wang Hai wrote:
> syzkaller reports for memory leak when kobject_init_and_add()
> returns an error in the function sysfs_slab_add() [1]
> 
> When this happened, the function kobject_put() is not called for the
> corresponding kobject, which potentially leads to memory leak.
> 
> This patch fixes the issue by calling kobject_put() even if
> kobject_init_and_add() fails.

I think this speaks to a deeper problem with kobject_init_and_add()
-- the need to call kobject_put() if it fails is not readily apparent
to most users.  This same bug appears in the first three users of
kobject_init_and_add() that I checked --
arch/ia64/kernel/topology.c
drivers/firmware/dmi-sysfs.c
drivers/firmware/efi/esrt.c
drivers/scsi/iscsi_boot_sysfs.c

Some do get it right --
arch/powerpc/kernel/cacheinfo.c
drivers/gpu/drm/ttm/ttm_bo.c
drivers/gpu/drm/ttm/ttm_memory.c
drivers/infiniband/hw/mlx4/sysfs.c

I'd argue that the current behaviour is wrong, that kobject_init_and_add()
should call kobject_put() if the add fails.  This would need a tree-wide
audit.  But somebody needs to do that anyway because based on my random
sampling, half of the users currently get it wrong.