linux-ext4 - Re: [PATCH v8 1/9] dax: fix NULL pointer dereference in __dax

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Date:	Tue, 12 Jan 2016 10:34:58 +0100
From:	Jan Kara <jack@...e.cz>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	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 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
> ---
>  fs/dax.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 7af8797..513bba5 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -624,6 +624,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
>  	}
>  
>  	memset(&bh, 0, sizeof(bh));
> +	bh.b_bdev = inode->i_sb->s_bdev;
>  	block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);
>  
>  	bh.b_size = PMD_SIZE;
> -- 
> 2.5.0
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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