linux-hardening - Re: [PATCH v3 net-next 5/8] mfd: zl3073x: Add functions to work with register mailboxes

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Message-ID: <ee0b1569-1c01-4e7b-b4a0-ec1de4634e89@redhat.com>
Date: Thu, 17 Apr 2025 16:18:39 +0200
From: Ivan Vecera <ivecera@...hat.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: netdev@...r.kernel.org, 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>,
 Andy Shevchenko <andy@...nel.org>, Andrew Morton
 <akpm@...ux-foundation.org>, Michal Schmidt <mschmidt@...hat.com>,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 net-next 5/8] mfd: zl3073x: Add functions to work with
 register mailboxes



On 17. 04. 25 3:22 odp., Andrew Lunn wrote:
>>>> +/*
>>>> + * Mailbox operations
>>>> + */
>>>> +int zl3073x_mb_dpll_read(struct zl3073x_dev *zldev, u8 index);
>>>> +int zl3073x_mb_dpll_write(struct zl3073x_dev *zldev, u8 index);
>>>> +int zl3073x_mb_output_read(struct zl3073x_dev *zldev, u8 index);
>>>> +int zl3073x_mb_output_write(struct zl3073x_dev *zldev, u8 index);
>>>> +int zl3073x_mb_ref_read(struct zl3073x_dev *zldev, u8 index);
>>>> +int zl3073x_mb_ref_write(struct zl3073x_dev *zldev, u8 index);
>>>> +int zl3073x_mb_synth_read(struct zl3073x_dev *zldev, u8 index);
>>>> +int zl3073x_mb_synth_write(struct zl3073x_dev *zldev, u8 index);
>>>
>>> I assume these are the only valid ways to access a mailbox?
>>>
>>> If so:
>>>
>>>> +static inline __maybe_unused int
>>>> +zl3073x_mb_read_ref_mb_mask(struct zl3073x_dev *zldev, u16 *value)
>>>> +{
>>>> +     __be16 temp;
>>>> +     int rc;
>>>> +
>>>> +     lockdep_assert_held(&zldev->mailbox_lock);
>>>> +     rc = regmap_bulk_read(zldev->regmap, ZL_REG_REF_MB_MASK, &temp,
>>>> +                           sizeof(temp));
>>>> +     if (rc)
>>>> +             return rc;
>>>> +
>>>> +     *value = be16_to_cpu(temp);
>>>> +     return rc;
>>>> +}
>>>
>>> These helpers can be made local to the core. You can then drop the
>>> lockdep_assert_held() from here, since the only way to access them is
>>> via the API you defined above, and add the checks in those API
>>> functions.
>>
>> This cannot be done this way... the above API just simplifies the
>> operation of read and write latch registers from/to mailbox.
>>
>> Whole operation is described in the commit description.
>>
>> E.g. read something about DPLL1
>> 1. Call zl3073x_mb_dpll_read(..., 1)
>>     This selects DPLL1 in the DPLL mailbox and performs read operation
>> and waits for finish
>> 2. Call zl3073x_mb_read_dpll_mode()
>>     This reads dpll_mode latch register
>>
>> write:
>> 1. Call zl3073x_mb_write_dpll_mode(...)
>>     This writes mode to dpll_mode latch register
>> 2. Call zl3073x_mb_dpll_read(..., 1)
>>     This writes all info from latch registers to DPLL1
>>
>> The point is that between step 1 and 2 nobody else cannot touch
>> latch_registers or mailbox select register and op semaphore.
> 
> Again, think about your layering. What does your lower level mailbox
> API look like? What does the MFD need to export for a safe API?
> 
> So maybe you need zl3073x_mb_read_u8(), zl3073x_mb_read_u16(),
> zl3073x_mb_read_u32(), as part of your mailbox API. These assert the
> lock is held.
> 
> You could even make zl3073x_mb_read_u8() validate the register is in
> the upper range, and that zl3073x_read_u8() the register is in the
> lower range.

Yes, this LGTM... Anyway if the MB API would provide reading and writing 
MBs at once then zl3073x_mb_{read,write}_u* are not necessary as the 
only caller of these functions is MFD itself and they would be called 
under MB API that holds the lock.

Ivan