linux-kernel - Re: [PATCH v4 1/8] efi: efivars: don't crash in efivar_set_variable{,

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Message-ID: <aF1CX2uWZ_KaMDVR@hovoldconsulting.com>
Date: Thu, 26 Jun 2025 14:51:43 +0200
From: Johan Hovold <johan@...nel.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Bjorn Andersson <andersson@...nel.org>,
	Maximilian Luz <luzmaximilian@...il.com>,
	Konrad Dybcio <konradybcio@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	Steev Klimaszewski <steev@...i.org>, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-efi@...r.kernel.org
Subject: Re: [PATCH v4 1/8] efi: efivars: don't crash in
 efivar_set_variable{,_locked} in r/o case

On Thu, Jun 26, 2025 at 02:03:44PM +0300, Dmitry Baryshkov wrote:
> On Thu, Jun 26, 2025 at 12:04:30PM +0200, Johan Hovold wrote:
> > On Wed, Jun 25, 2025 at 01:53:20AM +0300, Dmitry Baryshkov wrote:
> > > If efivar implementation doesn't provide write support, then calling
> > > efivar_set_variable() (e.g. when PM8xxx RTC driver tries to update the
> > > RTC offset) will crash the system. Prevent that by checking that
> > > set_variable callback is actually provided and fail with an
> > > EFI_WRITE_PROTECTED if it is not.
> > > 
> > > Fixes: 472831d4c4b2 ("efi: vars: Add thin wrapper around EFI get/set variable interface")
> > 
> > I don't think a fixes tag is warranted here as it currently appears to
> > be expected that the callers check if setvar is supported before calling
> > this helper (e.g. by calling efivar_supports_writes() as efivarfs does).
> 
> It is not documented as such. So, I think, we'd better not crash the
> callers.

You need to look at the backstory to determine that before jumping to
conclusions (e.g. start by looking at f88814cc2578 ("efi/efivars: Expose
RT service availability via efivars abstraction")).

> > So should perhaps be fixed in the RTC driver if we agree that supporting
> > read-only offsets is indeed something we want.
> > 
> > Are there any other current user that may possibly benefit from
> > something like this?
> 
> efi-pstore comes to my mind.

No, that driver is also disabled when efivar_supports_writes() returns
false.

Johan