linux-kernel - Re: [PATCH][RFC][resend] potential NULL pointer deref in XFS on failed mount

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Date:	Thu, 16 Nov 2006 23:18:00 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"David Chinner" <dgc@....com>
Cc:	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	xfs-masters@....sgi.com, "Andrew Morton" <akpm@...l.org>
Subject: Re: [PATCH][RFC][resend] potential NULL pointer deref in XFS on failed mount

On 16/11/06, David Chinner <dgc@....com> wrote:
> On Thu, Nov 16, 2006 at 10:18:26PM +0100, Jesper Juhl wrote:
> > (got no reply on this when I originally send it on 20061031, so resending
> >  now that a bit of time has passed.  The patch still applies cleanly to
> >  Linus' git tree as of today.)
> >
> >
> > The Coverity checker spotted a potential problem in XFS.
> >
> > The problem is that if, in xfs_mount(), this code triggers:
> >
> >       ...
> >       if (!mp->m_logdev_targp)
> >               goto error0;
> >       ...
> >
> > Then we'll end up calling xfs_unmountfs_close() with a NULL
> > 'mp->m_logdev_targp'.
> > This in turn will result in a call to xfs_free_buftarg() with its 'btp'
> > argument == NULL. xfs_free_buftarg() dereferences 'btp' leading to
> > a NULL pointer dereference and crash.
>
> Interesting that coverity found that, but failed to find the other
> leaks in that function from exactly the same code and error
> case.....
>
> > I think this can happen, since the fatal call to xfs_free_buftarg()
> > happens when 'm_logdev_targp != m_ddev_targp' and due to a check of
> > 'm_ddev_targp' against NULL in xfs_mount() (and subsequent return if it is
> > NULL) the two will never both be NULL when we hit the error0 label from
> > the two lines cited above.
> >
> > Comments welcome (please keep me on Cc: on replies).
> >
> > Here's a proposed patch to fix this by testing 'btp' against NULL in
> > xfs_free_buftarg().
>
> Not the right fix - we should only be trying to free valid
> buftargs, which means xfs_unmountfs_close() is the correct
> place to fix this....
>

Ok.

> e.g:
>
> -       if (mp->m_logdev_targp != mp->m_ddev_targp)
> +       if (mp->m_logdev_targp && (mp->m_logdev_targp != mp->m_ddev_targp))
>
> As to the afore-mentioned leaks, if we fail to allocate a realtime
> buftarg, then we will leak a reference to both the rtdev and logdev,
> and if we fail to allocate an external log buftarg we'll leak a
> reference to the logdev. i.e., we fail to do one or both of:
>
>         xfs_blkdev_put(logdev);
>         xfs_blkdev_put(rtdev);
>
> To remove the bdev references we may have gained earlier. Normally,
> these references are released by xfs_free_buftarg(), but because we
> failed to allocate the buftarg, we can't drop the references via
> that method....
>

Thanks a lot for commenting Dave.

I don't have time to look more at this tonight or tomorrow, but I'll
try to cook up a proper fix covering all cases during the weekend.

-- 
Jesper Juhl <jesper.juhl@...il.com>
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