linux-kernel - RE: [PATCH 1/7] fs, xfs: convert xfs_bui_log_item.bui_refcount from atomic_t to refcount

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Date:   Thu, 23 Feb 2017 07:50:15 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Dave Chinner <david@...morbit.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: RE: [PATCH 1/7] fs, xfs: convert xfs_bui_log_item.bui_refcount from
 atomic_t to refcount_t

> On Wed, Feb 22, 2017 at 11:20:31AM +0000, Reshetova, Elena wrote:
> > > On Tue, Feb 21, 2017 at 05:49:01PM +0200, Elena Reshetova wrote:
> > > > 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.
> > >
> > > I'm missing something: how do you overflow a log item object
> > > reference count?
> >
> > We are currently converting all reference counters present in kernel to a
> safer refcount_t type.
> 
> Yes, I see that you are taking anything that you *think* is an
> object lifetime reference counter and changing it.
> 
> > Agreed, in some cases it might be easier or harder to actually create/trigger
> an overflow, but since it can be caused even by a bug in the legitimate code
> (current version or its future iterative), it is good idea to do "safe defaults" and
> stop worrying about the problem.
> >
> > Do you have any reasons why it should not be converted?
> 
> It's core dirty metadata object code.  Any change to code in this
> area needs to be gone over with a fine tooth comb, because bugs can
> result in filesystem and/or journal corruption issues that may not
> be noticed until a system crashes and log recovery fails and the
> user loses their entire filesystem....
> 
> Hence the repeated comments about needing to actually test the code
> you are changing.

Sure, we are now in the process of testing this run-time as was suggested using xfstests. 
I will only repost this series after we done with testing and fix issues.
 
Best Regards,
Elena.


> 
> Cheers,
> 
> Dave.
> --
> Dave Chinner
> david@...morbit.com