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

[Jan Kara]

On Wed 06-01-16 11:14:09, Dan Williams wrote:
> 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?

No. The real problem is a long-standing abuse of struct buffer_head to be
used for passing block mapping information (it's on my todo list to remove
that at least from DAX code and use cleaner block mapping interface but
first I want basic DAX functionality to settle down to avoid unnecessary
conflicts). Filesystem is not supposed to touch bh->b_bdev. If you need
that filled in, set it yourself in before passing bh to the block mapping
function.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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