Re: [PATCH 2/3] soc: qcom: rpmh: Update rpm_msgs offset address and add list_del

[Evan Green <evgreen@chromium.org>]

On Tue, Feb 4, 2020 at 9:12 PM Maulik Shah <mkshah@codeaurora.org> wrote:
>
>
> On 2/5/2020 6:01 AM, Evan Green wrote:
> > On Mon, Feb 3, 2020 at 10:14 PM Maulik Shah <mkshah@codeaurora.org> wrote:
> >> rpm_msgs are copied in continuously allocated memory during write_batch.
> >> Update request pointer to correctly point to designated area for rpm_msgs.
> >>
> >> While at this also add missing list_del before freeing rpm_msgs.
> >>
> >> Signed-off-by: Maulik Shah <mkshah@codeaurora.org>
> >> ---
> >>   drivers/soc/qcom/rpmh.c | 9 ++++++---
> >>   1 file changed, 6 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/soc/qcom/rpmh.c b/drivers/soc/qcom/rpmh.c
> >> index c3d6f00..04c7805 100644
> >> --- a/drivers/soc/qcom/rpmh.c
> >> +++ b/drivers/soc/qcom/rpmh.c
> >> @@ -65,7 +65,7 @@ struct cache_req {
> >>   struct batch_cache_req {
> >>          struct list_head list;
> >>          int count;
> >> -       struct rpmh_request rpm_msgs[];
> >> +       struct rpmh_request *rpm_msgs;
> >>   };
> >>
> >>   static struct rpmh_ctrlr *get_rpmh_ctrlr(const struct device *dev)
> >> @@ -327,8 +327,10 @@ static void invalidate_batch(struct rpmh_ctrlr *ctrlr)
> >>          unsigned long flags;
> >>
> >>          spin_lock_irqsave(&ctrlr->cache_lock, flags);
> >> -       list_for_each_entry_safe(req, tmp, &ctrlr->batch_cache, list)
> >> +       list_for_each_entry_safe(req, tmp, &ctrlr->batch_cache, list) {
> >> +               list_del(&req->list);
> >>                  kfree(req);
> >> +       }
> >>          INIT_LIST_HEAD(&ctrlr->batch_cache);
> > Hm, I don't get it. list_for_each_entry_safe ensures you can traverse
> > the list while freeing it behind you. ctrlr->batch_cache is now a
> > bogus list, but is re-inited with the lock held. From my reading,
> > there doesn't seem to be anything wrong with the current code. Can you
> > elaborate on the bug you found?
>
> Hi Evan,
>
> when we don't do list_del, there might be access to already freed memory.
> Even after current item free via kfree(req), without list_del, the next
> and prev item's pointer are still pointing to this freed region.
> it seem best to call list_del to ensure that before freeing this area,
> no other item in list refer to this.

I don't think that's true. the "_safe" part of
list_for_each_entry_safe ensures that we don't touch the ->next member
of any node after freeing it. So I don't think there's any case where
we could touch freed memory. The list_del still seems like needless
code to me.

>
> >
> >>          spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
> >>   }
> >> @@ -377,10 +379,11 @@ int rpmh_write_batch(const struct device *dev, enum rpmh_state state,
> >>                  return -ENOMEM;
> >>
> >>          req = ptr;
> >> +       rpm_msgs = ptr + sizeof(*req);
> >>          compls = ptr + sizeof(*req) + count * sizeof(*rpm_msgs);
> >>
> >>          req->count = count;
> >> -       rpm_msgs = req->rpm_msgs;
> >> +       req->rpm_msgs = rpm_msgs;
> > I don't really understand what this is fixing either, can you explain?
> the continous memory allocated via below is for 3 items,
>
> ptr = kzalloc(sizeof(*req) + count * (sizeof(req->rpm_msgs[0]) +
> sizeof(*compls)), GFP_ATOMIC);
>
> 1. batch_cache_req,  followed by
> 2. total count of rpmh_request,  followed by
> 3. total count of compls
>
> current code starts using (3) compls from proper offset in memory
>          compls = ptr + sizeof(*req) + count * sizeof(*rpm_msgs);
>
> however for (2) rpmh_request it does
>
>          rpm_msgs = req->rpm_msgs;
>
> because of this it starts 8 byte before its designated area and overlaps
> with (1) batch_cache_req struct's last entry.
> this patch corrects it via below to ensure rpmh_request uses correct
> start address in memory.
>
>          rpm_msgs = ptr + sizeof(*req);

I don't follow that either. The empty array declaration (or the
GCC-specific version of it would be  "struct rpmh_request
rpm_msgs[0];") is a flexible array member, meaning the member itself
doesn't take up any space in the struct. So, for instance, it holds
true that &(req->rpm_msgs[0]) == (req + 1). By my reading the existing
code is correct, and your patch just adds a needless pointer
indirection. Check out this wikipedia entry:

https://en.wikipedia.org/wiki/Flexible_array_member