Re: kobject_init_and_add is easy to misuse

[Konstantin Khlebnikov]

On 02/06/2020 15.10, Matthew Wilcox wrote:
> 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.

At his point kobject doesn't own kmem-cache structure itself yet.

So calling kobject_put() will free kmem-cache and then it will be
freed second time on error path in create_cache().

I suppose freeing in case of error should be pushed from common
create_cache() into slab-specific __kmem_cache_create().