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

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Message-ID: <8802b276-b6dd-4235-87dd-18b835edb196@lunn.ch>
Date: Thu, 17 Apr 2025 15:22:12 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Ivan Vecera <ivecera@...hat.com>
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

> > > +/*
> > > + * 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.

	Andrew