Date:   Wed, 12 Feb 2020 17:45:38 +0530
From:   Maulik Shah <mkshah@...eaurora.org>
To:     Evan Green <evgreen@...omium.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Andy Gross <agross@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Lina Iyer <ilina@...eaurora.org>, lsrao@...eaurora.org
Subject: Re: [PATCH 2/3] soc: qcom: rpmh: Update rpm_msgs offset address and
 add list_del

On 2/5/2020 11:51 PM, Evan Green wrote:
> On Tue, Feb 4, 2020 at 9:12 PM Maulik Shah <mkshah@...eaurora.org> wrote:
>>
>> On 2/5/2020 6:01 AM, Evan Green wrote:
>>> On Mon, Feb 3, 2020 at 10:14 PM Maulik Shah <mkshah@...eaurora.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@...eaurora.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.

Hmm, ok. i can drop list_del.

see the reason below to include list_del.

>>>>           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
Thanks Evan,

Agree that code works even without this.

However from the same wiki,

 >>It is common to allocate sizeof(struct) + array_len*sizeof(array 
element) bytes.

 >>This is not wrong, however it may allocate a few more bytes than 
necessary:

this is what i wanted to convery above, currently it allocated 8 more 
bytes than necessary.

The reason for the change was one use after free reported in rpmh driver.

After including this change, we have not seen this reported again.

I can drop this change in new revision if we don't want it.

Thanks,

Maulik

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