linux-hardening - Re: [PATCH 01/28] mfd: Add Microchip ZL3073x support

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Message-ID: <ad5ada81-d611-41bb-8358-3675f90767f1@redhat.com>
Date: Mon, 14 Apr 2025 16:53:33 +0200
From: Ivan Vecera <ivecera@...hat.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: Andy Shevchenko <andy@...nel.org>, netdev@...r.kernel.org,
 Michal Schmidt <mschmidt@...hat.com>,
 Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
 Arkadiusz Kubalewski <arkadiusz.kubalewski@...el.com>,
 Jiri Pirko <jiri@...nulli.us>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Prathosh Satish <Prathosh.Satish@...rochip.com>,
 Lee Jones <lee@...nel.org>, Kees Cook <kees@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 01/28] mfd: Add Microchip ZL3073x support



On 14. 04. 25 4:16 odp., Andy Shevchenko wrote:
> On Mon, Apr 14, 2025 at 5:13 PM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
>> On Mon, Apr 14, 2025 at 5:10 PM Andy Shevchenko
>> <andy.shevchenko@...il.com> wrote:
>>> On Mon, Apr 14, 2025 at 5:07 PM Ivan Vecera <ivecera@...hat.com> wrote:
>>>> On 14. 04. 25 1:52 odp., Ivan Vecera wrote:
> 
> ...
> 
>>>> Long story short, I have to move virtual range outside real address
>>>> range and apply this offset in the driver code.
>>>>
>>>> Is this correct?
>>>
>>> Bingo!
>>>
>>> And for the offsets, you form them as "page number * page offset +
>>> offset inside the page".
>>
>> Note, for easier reference you may still map page 0 to the virtual
>> space, but make sure that page 0 (or main page) is available outside
>> of the ranges, or i.o.w. ranges do not overlap the main page, even if
>> they include page 0.
> 
> So, you will have the following layout
> 
> 0x00 - 0xnn - real registers of page 0.
> 
> 0x100 - 0xppp -- pages 0 ... N
> 
> Register access either direct for when direct is required, or as
> 0x100 + PageSize * Index + RegOffset

Now, get it...

I was a little bit confused by code of _regmap_select_page() that takes 
care of selector_reg.

Btw, why is this needed? why they cannot overlap?

Let's say I have virtual range <0, 0xfff>, window <0, 0xff> and window 
selector 0xff>.
1. I'm calling regmap_read(regmap, 0x8f, ...)
2. The regmap looks for the range and it finds it (0..0xfff)
3. Then it calls _regmap_select_page() that computes:
    window_offset = (0x8f - 0x000) % 0x100 = 0x8f
    window_page = (0x8f - 0x000) / 0x100 = 0
4. _regmap_select_page() set window selector to 0 and reg is updated to
    reg = window_start + window_offset = 0x8f

And for window_selector value: regmap_read(regmap, 0xff, ...) is the 
same except _regmap_select_page() checks that the given address is 
selector_reg and won't perform page switching.

When I think about it, in my case there is no normal page, there is only 
volatile register window <0x00-0x7e> and only single direct register 
that is page selector at 0x7f.

Thanks,
Ivan