linux-kernel - Re: [PATCH V1 01/15] spmi: pmic_arb: block access of invalid read and writes

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Date:   Wed, 14 Jun 2017 20:39:19 +0530
From:   kgunda@...eaurora.org
To:     Stephen Boyd <sboyd@...eaurora.org>
Cc:     Abhijeet Dharmapurikar <adharmap@...eaurora.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        David Collins <collinsd@...eaurora.org>,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        adharmap@...cinc.com, aghayal@....qualcomm.com,
        linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCH V1 01/15] spmi: pmic_arb: block access of invalid read and
 writes

On 2017-06-13 07:39, Stephen Boyd wrote:
> On 06/12, kgunda@...eaurora.org wrote:
>> On 2017-05-31 06:03, Stephen Boyd wrote:
>> >On 05/30, Kiran Gunda wrote:
>> >>From: Abhijeet Dharmapurikar <adharmap@...eaurora.org>
>> >>
>> >>The system crashes due to bad access when reading from an non
>> >>configured
>> >>peripheral and when writing to peripheral which is not owned by
>> >>current
>> >>ee. This patch verifies ownership to avoid crashing on
>> >>write.
>> >
>> >What systems? As far as I know we don't have any bad accesses
>> >happening right now. If they are happening, we should fix the
>> >code that's accessing hardware that isn't owned by them.
>> >
>> This change greatly improves the debugging effort for developers by
>> printing
>> a very simple and clear error message when an invalid SPMI access 
>> occurs
>> (due to bad DT configuration, bad bootloader SPMI permission
>> configurations,
>> or other issues).  Without this change, such accesses will cause XPU
>> violations
>> that crash the system and require extensive effort to decode.
> 
> Right, but they're easily detectable because we would know almost
> immediately that something isn't working when we integrate a
> change. If you update the DT and it stops working, the DT is bad.
> If you update the bootloader and it stops working, the bootloader
> is bad, etc.
> 
Ok. Will send a patch to remove this code in the next series.
>> 
>> >>For reads, since the forward mapping table, data_channel->ppid, is
>> >>towards the end of the block, we use the core size to figure the
>> >>max number of ppids supported. The table starts at an offset of 0x800
>> >>within the block, so size - 0x800 will give us the area used by the
>> >>table. Since each table is 4 bytes long (core_size - 0x800) / 4 will
>> >>gives us the number of data_channel supported.
>> >>This new protection is functional on hw v2.
>> >
>> >Which brings us to the next question which is why do we need this
>> >patch at all? We aren't probing hardware to see what we have
>> >access to and then populating device structures based on that.
>> >Instead, we're just populating DT nodes that we've hardcoded in
>> >the dts files, so I'm a little lost on why we would have a node
>> >in there that we couldn't access. Please add such details to the
>> >commit text.
>> >
>> invalid SPMI access occurs due to bad DT configuration, bad
>> bootloader SPMI
>> permission configurations, or other issues. This change reduces the
>> debugging
>> effort for developers by printing clear error message when an
>> invalid SPMI
>> access occurs.
> 
> Well we also take an overhead on every read/write. Sure things
> are slow so the overhead is negligible, but the permissions are
> on a peripheral id basis, so really we should look into _not_
> populating devices that aren't accessible in the first place.
> Then we move the checks out of the read/write path and to a more
> logical place whereby we prevent a driver from attempting to even
> attach to read or write a register that is protected.
Ok. Will remove this code in the next patch series and try to implement 
it
as per your suggestion.