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

[Ross Zwisler]

On Fri, Jan 08, 2016 at 10:10:00AM +1100, Dave Chinner wrote:
> On Thu, Jan 07, 2016 at 10:34:02AM +0100, Jan Kara wrote:
> > 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.
> 
> That has not been true for a long, long time. e.g. XFS always
> rewrites bh->b_bdev in get_blocks because the file may not reside on
> the primary block device of the filesystem. i.e.:
> 
>         /*
>          * If this is a realtime file, data may be on a different device.
>          * to that pointed to from the buffer_head b_bdev currently.
>          */
>         bh_result->b_bdev = xfs_find_bdev_for_inode(inode);
> 
> > If you need
> > that filled in, set it yourself in before passing bh to the block mapping
> > function.
> 
> That may be true, but we cannot assume that the bdev coming back
> out of get_block is the same one that was passed in.

For our use case I think this is fine - we just need the bdev to be filled in
so that we can print reasonable error messages.  If the filesystem updates
bh->b_bdev during get_blocks(), we are fine with that.