Re: [Xen-devel] [PATCH 08/29] drivers, md: convert mddev.active from atomic_t to refcount_t

["Reshetova, Elena" <elena.reshetova@xxxxxxxxx>]

> On Tue, 2017-03-14 at 12:29 +0000, Reshetova, Elena wrote:
> > > Elena Reshetova <elena.reshetova@xxxxxxxxx> writes:
> > >
> > > > refcount_t type and corresponding API should be
> > > > used instead of atomic_t when the variable is used as
> > > > a reference counter. This allows to avoid accidental
> > > > refcounter overflows that might lead to use-after-free
> > > > situations.
> > > >
> > > > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> > > > Signed-off-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> > > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > > Signed-off-by: David Windsor <dwindsor@xxxxxxxxx>
> > > > ---
> > > >  drivers/md/md.c | 6 +++---
> > > >  drivers/md/md.h | 3 ++-
> > > >  2 files changed, 5 insertions(+), 4 deletions(-)
> > >
> > > When booting linux-next (specifically 5be4921c9958ec) I'm seeing
> > > the
> > > backtrace below. I suspect this patch is just exposing an existing
> > > issue?
> >
> > Yes, we have actually been following this issue in the another
> > thread.
> > It looks like the object is re-used somehow, but I can't quite
> > understand how just by reading the code.
> > This was what I put into the previous thread:
> >
> > "The log below indicates that you are using your refcounter in a bit
> > weird way in mddev_find().
> > However, I can't find the place (just by reading the code) where you
> > would increment refcounter from zero (vs. setting it to one).
> > It looks like you either iterate over existing nodes (and increment
> > their counters, which should be >= 1 at the time of increment) or
> > create a new node, but then mddev_init() sets the counter to 1. "
> >
> > If you can help to understand what is going on with the object
> > creation/destruction, would be appreciated!
> >
> > Also Shaohua Li stopped this patch coming from his tree since the
> > issue was caught at that time, so we are not going to merge this
> > until we figure it out.
> 
> Asking on the correct list (dm-devel) would have got you the easy
> answer:  The refcount behind mddev->active is a genuine atomic.  It has
> refcount properties but only if the array fails to initialise (in that
> case, final put kills it).  Once it's added to the system as a gendisk,
> it cannot be freed until md_free().  Thus its ->active count can go to
> zero (when it becomes inactive; usually because of an unmount). On a
> simple allocation regardless of outcome, the last executed statement in
> md_alloc is mddev_put(): that destroys the device if we didn't manage
> to create it or returns 0 and adds an inactive device to the system
> which the user can get with mddev_find().

Thank you James for explaining this! I guess in this case, the conversion 
doesn't make sense. 
And sorry about not asking in a correct place: we are handling many similar 
patches now and while I try to reach the right audience using get_maintainer 
script, it doesn't always succeeds. 

Best Regards,
Elena.

> 
> James
> 

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