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

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

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.