Re: [PATCH v8 1/9] dax: fix NULL pointer dereference in __dax_dbg()

[Jan Kara]

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@xxxxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>

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@xxxxxxxx>
SUSE Labs, CR