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

[Ross Zwisler]

On Thu, Jan 07, 2016 at 07:17:22AM -0800, Dan Williams wrote:
> On Thu, Jan 7, 2016 at 1:34 AM, Jan Kara <jack@xxxxxxx> 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. If you need
> > that filled in, set it yourself in before passing bh to the block mapping
> > function.
> >
> 
> Ok, makes sense.
> 
> Ross, can you fix this instead by unconditionally looking up the bdev
> rather that saying "unknown".  The bdev should always be retrievable.

Sure, will do.