linux-kernel - Re: kobject_init_and

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Message-ID: <20200602145721.GM19604@bombadil.infradead.org>
Date:   Tue, 2 Jun 2020 07:57:21 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Wang Hai <wanghai38@...wei.com>, 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
Subject: Re: kobject_init_and_add is easy to misuse

On Tue, Jun 02, 2020 at 04:04:04PM +0200, Greg Kroah-Hartman wrote:
> 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...

There's around 120 callers in the kernel today ... large, indeed.

> 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 :)

Yes ;-)

> > 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.

My audit may not be correct then.  The kobject_put() may be appropriately
being called at a higher level rather than in the same function as the
kobject_init_and_add().