linux-kernel - Re: [PATCH] stm class: Fix out of bound access from bitmap allocation

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Message-ID: <f3a699e5-a6c4-83f4-fff3-20515e8caa06@arm.com>
Date:   Mon, 8 Apr 2019 11:52:23 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     David Laight <David.Laight@...LAB.COM>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mulu He <muluhe@...eaurora.org>,
        Tingwei Zhang <tingwei@...eaurora.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>
Cc:     Rajendra Nayak <rnayak@...eaurora.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Sibi Sankar <sibis@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] stm class: Fix out of bound access from bitmap allocation

On 08/04/2019 11:33, David Laight wrote:
> From: Robin Murphy
>> Sent: 08 April 2019 11:24
>> On 07/04/2019 05:31, Sai Prakash Ranjan wrote:
>>> On 4/5/2019 6:44 PM, David Laight wrote:
>>>> From: Sai Prakash Ranjan
>>>>>
>>>>> diff --git a/drivers/hwtracing/stm/core.c b/drivers/hwtracing/stm/core.c
>>>>> index 93ce3aa740a9..21a5838f6e67 100644
>>>>> --- a/drivers/hwtracing/stm/core.c
>>>>> +++ b/drivers/hwtracing/stm/core.c
>>>>> @@ -168,7 +168,7 @@ static int stp_master_alloc(struct stm_device
>>>>> *stm, unsigned int idx)
>>>>>        struct stp_master *master;
>>>>>        size_t size;
>>>>>
>>>>> -    size = ALIGN(stm->data->sw_nchannels, 8) / 8;
>>>>> +    size = ALIGN(stm->data->sw_nchannels, STM_MASTER_SZ) /
>>>>> STM_MASTER_SZ;
>>>>
>>>> I'm not sure that using STP_MASTER_SZ improves readability at all.
>>>>
>>>
>>> I thought it was better to have a macro than directly specifying
>>> sizeof(unsigned long), anyways I can change it.
>>>
>>>> Is there something that gives the size of a bitmap for 'n' items?
>>>>
>>>
>>> Not sure if there is something.
>>
>> If you were to ask the question "how does the bitmap code itself know
>> what the total size of a bitmap is?", that would quickly lead you
>> towards BITS_TO_LONGS() ;)
>>
>> And given that stp_master::chan_map is already an appropriate type, that
>> suggests simplifying the entire calculation down to something neat and
>> tidy like:
>>
>> size = offsetof(struct stp_master, chan_map[BITS_TO_LONGS(stm->data->sw_nchannels)]);
> 
> Except that is invalid.
> You can't use offsetof() with something that isn't a compile time constant.

Oh, I see the standard does actually say that, although there seem to be 
enough non-constant uses in the kernel to suggest that it still works in 
practice.

However, while writing the above I was still trying to remember the 
other thing I'd seen for handling precisely this variable-sized-struct 
situation, which I've now found again, namely struct_size(). I guess now 
I understand why that isn't implemented in terms of offsetof(), thanks 
for the nudge :)

Robin.