Re: kobject_init_and_add is easy to misuse

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2020-06-02 at 19:36 +0200, Greg Kroah-Hartman wrote:
> On Tue, Jun 02, 2020 at 08:25:14AM -0700, James Bottomley wrote:
> > On Tue, 2020-06-02 at 05:10 -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
> > > 
> > > I'd argue that the current behaviour is wrong,
> > 
> > Absolutely agree with this.  We have a big meta pattern here where
> > we introduce functions with tortuous semantics then someone creates
> > a checker for the semantics and misuses come crawling out of the
> > woodwork leading to floods of patches, usually for little or never
> > used error paths, which really don't buy anything apart from
> > theoretical correctness.  Just insisting on simple semantics would
> > have avoided this.
> 
> I "introduced" this way back at the end of 2007.  It's not exactly a
> new function.

Heh, well, if it never fails, how you handle the failure become
unimportant semantics ...

> > >  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.
> > 
> > Well, the semantics of kobject_init() are free on fail, so these
> > are the ones everyone seems to be using.  The semantics of
> > kobject_add are put on fail.  The problem is that put on fail isn't
> > necessarily correct in the kobject_init() case: the release
> > function may make assumptions about the object hierarchy which
> > aren't satisfied in the kobject_init() failure case.  This argues
> > that kobject_init_and_add() can't ever have correct semantics and
> > we should eliminate it.
> 
> At the time, it did reduce common functionality and error handling
> all into a simpler function.  And, given it's history, it must have
> somehow worked for the past 12 years or so :)

Well, like I said, as long as it never fails, no problem.

It was just Matthew saying "couldn't we make it do kobject_put()
itself?" that got me thinking that perhaps that wouldn't work with all
cases.  So now we're discussing failure handling, we're into the
esoteric rabbit hole case that never happens.

> Odds are, lots of the callers shouldn't be messing around with
> kobjects in the first place.  Originally it was only assumed that
> there would be very few users.  But it has spread to filesystems and
> firmware subsystems.  Drivers should never use it though, so it's a
> good hint something is wrong there...
> 
> Anyway, patches to fix this up to make a "sane" api for kobjects is
> always appreciated.  Personally I don't have the time at the moment.

I think the only way we can make the failure semantics consistent is to
have the kobject_init() ones (so kfree on failure).  That means for the
add part, the function would have to unwind everything it did from init
on so kfree() is still an option.  If people agree, then I can produce
the patch ... it's just the current drive to transform everyone who's
doing kfree() into kobject_put() would become wrong ...

James