kobject_init_and_add is easy to misuse

[Matthew Wilcox]

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.