linux-kernel - Re: [PATCH v8 1/9] dax: fix NULL pointer dereference in __dax

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Date:	Wed, 13 Jan 2016 10:07:34 +0100
From:	Jan Kara <jack@...e.cz>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Theodore Ts'o <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvdimm@...ts.01.org, x86@...nel.org,
	xfs@....sgi.com
Subject: Re: [PATCH v8 1/9] dax: fix NULL pointer dereference in __dax_dbg()

On Wed 13-01-16 00:08:29, Ross Zwisler wrote:
> On Tue, Jan 12, 2016 at 10:34:58AM +0100, Jan Kara wrote:
> > On Thu 07-01-16 22:27:51, Ross Zwisler wrote:
> > > In __dax_pmd_fault() we currently assume that get_block() will always set
> > > bh.b_bdev and we unconditionally dereference it in __dax_dbg().  This
> > > assumption isn't always true - when called for reads of holes
> > > ext4_dax_mmap_get_block() returns a buffer head where bh->b_bdev is never
> > > set.  I hit this BUG while testing the DAX PMD fault path.
> > > 
> > > Instead, initialize bh.b_bdev before passing bh into get_block().  It is
> > > possible that the filesystem's get_block() will update bh.b_bdev, and this
> > > is fine - we just want to initialize bh.b_bdev to something reasonable so
> > > that the calls to __dax_dbg() work and print something useful.
> > > 
> > > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > > Cc: Dan Williams <dan.j.williams@...el.com>
> > 
> > Looks good. But don't you need to do the same for __dax_fault(),
> > dax_zero_page_range() and similar places passing bh to dax functions?
> > 
> > 								Honza
> 
> I don't think we need it anywhere else.  The only reason that we need to
> initialize the bh.b_bdev manually in the __dax_pmd_fault() path is that if the
> get_block() call ends up finding a hole (so doesn't fill out b_bdev) we still
> go through the dax_pmd_dbg() path to print an error message, which uses
> b_bdev.  I believe that in the other paths where we hit a hole, such as
> __dax_fault(), we don't use b_bdev because we don't have the same error path
> prints, and the regular code for handling holes doesn't use b_bdev.
> 
> That being said, if you feel like it's cleaner to initialize it
> everywhere so everything is consistent and we don't have to worry about
> it, I'm fine to make the change.

Well, it seems more futureproof to me. In case someone decides to add some
debug message later on...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR