Re: kobject_init_and_add is easy to misuse

From: Konstantin Khlebnikov
Date: Tue Jun 02 2020 - 09:48:58 EST

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 --

Some do get it right --

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().