Re: [PATCH v7 1/9] dax: fix NULL pointer dereference in __dax_dbg()

[Dan Williams <dan.j.williams@xxxxxxxxx>]

On Wed, Jan 6, 2016 at 10:00 AM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> __dax_dbg() currently assumes that bh->b_bdev is non-NULL, passing it into
> bdevname() where is is dereferenced.  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, verify that we have a valid bh->b_bdev, else just say "unknown"
> for the block device.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  fs/dax.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 7af8797..03cc4a3 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -563,7 +563,12 @@ static void __dax_dbg(struct buffer_head *bh, unsigned 
> long address,
>  {
>         if (bh) {
>                 char bname[BDEVNAME_SIZE];
> -               bdevname(bh->b_bdev, bname);
> +
> +               if (bh->b_bdev)
> +                       bdevname(bh->b_bdev, bname);
> +               else
> +                       snprintf(bname, BDEVNAME_SIZE, "unknown");
> +
>                 pr_debug("%s: %s addr: %lx dev %s state %lx start %lld "
>                         "length %zd fallback: %s\n", fn, current->comm,
>                         address, bname, bh->b_state, (u64)bh->b_blocknr,

I'm assuming there's no danger of a such a buffer_head ever being used
for the bdev parameter to dax_map_atomic()?  Shouldn't we also/instead
go fix ext4 to not send partially filled buffer_heads?