Re: [PATCH V2] i40iw: Do not set self-referencing pointer to NULL after kfree

Shiraz Saleem
2016-08-23T19:00:22Z

On Tue, Aug 23, 2016 at 09:46:23PM +0300, Leon Romanovsky wrote:
> On Tue, Aug 23, 2016 at 12:51:10PM -0400, Doug Ledford wrote:
> > On 8/23/2016 12:00 PM, Shiraz Saleem wrote:
> > > On Tue, Aug 23, 2016 at 04:41:35AM +0300, Leon Romanovsky wrote:
> > >> On Mon, Aug 22, 2016 at 07:01:47PM -0500, Shiraz Saleem wrote:
> > >>> From: Mustafa Ismail <mustafa.ismail-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> > >>>
> > >>> In i40iw_free_virt_mem(), do not set mem->va to NULL
> > >>> after freeing it as mem->va is a self-referencing pointer
> > >>> to mem.
> > >>
> > >> Sorry, I failed to understand your commit message and your change.
> > >> What did you mean? How do you suppose to use mem->va pointer
> > >> after kfree() call on that pointer? Won't you have use-after-free bug in
> > >> such case?
> > >
> > > The pointer mem->va cannot be used after kfree. But setting it to
> > > NULL would be writing to freed memory. In i40iw_puda_alloc_buf(),
> > > when a buffer is allocated of type struct i40iw_puda_buf, the address
> > > of the buffer itself is stored within the structure (in member buf_mem).
> > > When this pointer is freed, the structure containing the pointer is freed,
> > > so writing to the structure would be writing to freed memory, which is
> > > what this fix is for.
> > >
> > >>>
> > >>> Fixes: 4e9042e647ff ("i40iw: add hw and utils files")
> > >>>
> > >>> Reported-by: Stefan Assmann <sassmann-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> > >>> Signed-off-by: Mustafa Ismail <mustafa.ismail-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> > >>> Signed-off-by: Shiraz Saleem <shiraz.saleem-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> > >>> ---
> > >>>
> > >>> V2: Fix typo in subject line.
> > >>>
> > >>> drivers/infiniband/hw/i40iw/i40iw_utils.c | 1 -
> > >>> 1 file changed, 1 deletion(-)
> > >>>
> > >>> diff --git a/drivers/infiniband/hw/i40iw/i40iw_utils.c b/drivers/infiniband/hw/i40iw/i40iw_utils.c
> > >>> index 0e8db0a..d5f5de2 100644
> > >>> --- a/drivers/infiniband/hw/i40iw/i40iw_utils.c
> > >>> +++ b/drivers/infiniband/hw/i40iw/i40iw_utils.c
> > >>> @@ -674,7 +674,6 @@ enum i40iw_status_code i40iw_free_virt_mem(struct i40iw_hw *hw,
> > >>> if (!mem)
> > >>> return I40IW_ERR_PARAM;
> > >>> kfree(mem->va);
> > >>> - mem->va = NULL;
> > >>> return 0;
> >
> > Your commit message is a bit hard to follow, but if I follow the
> > conversation, kfree(mem->va) is the same as kfree(mem), is it not? If
> > it is, couldn't you just kfree(mem)? That would avoid the confusion
> > here. Also, if it matters, you can use a temporary pointer, aka:
> >
> > p = mem->va;
> > mem->va = NULL;
> > kfree(p);
> >
> > but, again, if mem->va is just a self-referencing pointer back to mem,
> > then why not just kfree(mem)? I'm concerned that this convoluted way of
> > doing things will come back to haunt us later when people think they are
> > submitting a fix and simply reintroduce the same bug again.
>
> Yeah, I totally agree with you.
>

OK, I see the the possible confusion of the commit message.

Maybe the following is clearer:
"In i40iw_free_virt_mem(), do not set mem->va to NULL after freeing it, as mem->va is a pointer
to the structure containing mem"

So kfree(mem->va) is not the same as kfree(mem).

--
To unsubscribe from this list: send the line "unsubscribe linux-rdma" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at http://vger.kernel.org/majordomo-info.html