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

Leon Romanovsky
2016-08-23T18:46:23Z

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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.

>
>
> --
> Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> GPG Key ID: 0E572FDD
>

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