Re: [Qemu-devel] [PATCH v3 3/8] block: Null pointer dereference in blk_root_get_parent_desc()

[Liam Merwick]

On 12/10/18 15:48, Max Reitz wrote:
> Hi,
>
> On 31.08.18 20:16, Liam Merwick wrote:
> > The dev_id returned by the call to blk_get_attached_dev_id() in
> > blk_root_get_parent_desc() can be NULL (an internal call to
> > object_get_canonical_path may have returned NULL) so it should
> > be checked before dereferencing.
> >
> > Signed-off-by: Liam Merwick <address@hidden>
> > Reviewed-by: Darren Kenny <address@hidden>
> > Reviewed-by: Mark Kanda <address@hidden>
> > ---
> >   block/block-backend.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/block/block-backend.c b/block/block-backend.c
> > index fa120630be83..210eee75006a 100644
> > --- a/block/block-backend.c
> > +++ b/block/block-backend.c
> > @@ -136,7 +136,7 @@ static char *blk_root_get_parent_desc(BdrvChild *child)
> >       }
> >   
> >       dev_id = blk_get_attached_dev_id(blk);
> > -    if (*dev_id) {
> > +    if (dev_id && *dev_id) {
> >           return dev_id;
>
> I rather think that blk_get_attached_dev_id() needs attention first.  It
> returns an explicitly empty string if blk->dev is NULL.  If NULL was a
> valid return value, it should just return NULL there.
>
> Besides this caller, there are two callers that pass the dev_id to
> qapi_event_send_device_tray_moved().  Now in practice that allows the
> string to be NULL, but there is a comment in visit_type_str() that says
> one should not pass NULL.
>
> So it's either changing blk_get_attached_dev_id() to return NULL when
> there is no valid ID (instead of the empty string, and then we could
> save ourselves the check "*dev_id" here and elsewhere), but then we have
> to fix all callers.
>
> Or we make it return an empty string if object_get_canonical_path()
> returned NULL.

I went with the latter and now (in upcoming v4) check the return value 
from object_get_canonical_path() and return an empty string if it's NULL.

Regards,
Liam


> Max
>
> >       } else {
> >           /* TODO Callback into the BB owner for something more detailed */