Re: [PATCH] accel/qaic: Fix dereferencing freed memory

[Jeffrey Hugo]

On 6/12/2023 7:21 AM, Christian König wrote:
> Am 12.06.23 um 15:03 schrieb Pranjal Ramajor Asha Kanojiya:
> > On 6/12/2023 4:52 PM, Christian König wrote:
> > > Am 10.06.23 um 04:12 schrieb Sukrut Bellary:
> > > > smatch warning:
> > > >     drivers/accel/qaic/qaic_data.c:620 qaic_free_object() error:
> > > >         dereferencing freed memory 'obj->import_attach'
> > > >
> > > > obj->import_attach is detached and freed using dma_buf_detach().
> > > > But used after free to decrease the dmabuf ref count using
> > > > dma_buf_put().
> > > >
> > > > Fixes: ff13be830333 ("accel/qaic: Add datapath")
> > > > Signed-off-by: Sukrut Bellary <sukrut.bellary@xxxxxxxxx>
> > > > ---
> > > >   drivers/accel/qaic/qaic_data.c | 4 +++-
> > > >   1 file changed, 3 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/accel/qaic/qaic_data.c 
> > > > b/drivers/accel/qaic/qaic_data.c
> > > > index e42c1f9ffff8..7cba4d680ea8 100644
> > > > --- a/drivers/accel/qaic/qaic_data.c
> > > > +++ b/drivers/accel/qaic/qaic_data.c
> > > > @@ -613,11 +613,13 @@ static int qaic_gem_object_mmap(struct 
> > > > drm_gem_object *obj, struct vm_area_struc
> > > >   static void qaic_free_object(struct drm_gem_object *obj)
> > > >   {
> > > >       struct qaic_bo *bo = to_qaic_bo(obj);
> > > > +    struct dma_buf *dmabuf;
> > >
> > > Maybe move that variable into the if.
> > >
> > > >       if (obj->import_attach) {
> > > >           /* DMABUF/PRIME Path */
> > > > +        dmabuf = obj->import_attach->dmabuf;
> > > >           dma_buf_detach(obj->import_attach->dmabuf, 
> > > > obj->import_attach);
> > > > -        dma_buf_put(obj->import_attach->dmabuf);
> > > > +        dma_buf_put(dmabuf);
> > >
> > > I strongly assume you are not using the GEM prime helpers for this?
> > >
> > > Christian.
> >
> > Driver uses drm_gem_prime_fd_to_handle() helper function but it also 
> > registers for ->gem_prime_import() which is internally called by 
> > drm_gem_prime_fd_to_handle(). All the operations done in 
> > gem_prime_import() are undone here.
>
> Then why don't you use drm_prime_gem_destroy() which is the cleanup 
> helper for imports created by ->gem_prime_import() ?
>
> That looks pretty much identical to what you do here manually.

I think destroy() wasn't used because we are new to DRM and sometimes 
confused by the multitude of options.  I appreciate the suggestion and 
will follow up to see if destroy() will work here, or identify what 
would prevent the use of it.

-Jeff