Re: kobject_init_and_add is easy to misuse

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Jun 02, 2020 at 05:10:35AM -0700, 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

Why are random individual drivers calling kobject* functions?  That
speaks to a larger problem here...

Anyway, yes, it's a tricky function, but the issue usually is that the
kobject is embedded in something else and if you call init_and_add() you
want to tear things down _before_ the final put happens.

The good thing is, that function is really hard to get to fail except if
you abuse it with syzkaller :)

> 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.

As said above, this is "tricky", and might break things.

thanks,

greg k-h