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

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Message-ID: <zw5u5c2itmpxq34d22y5wmtr32d4zsmjj5clf77ryeqs5jgd4v@t3wjfyj43yra>
Date: Thu, 26 Jun 2025 14:03:44 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Johan Hovold <johan@...nel.org>
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 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.

> 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.

> 
> > Reported-by: Johan Hovold <johan@...nel.org>
> > Closes: https://lore.kernel.org/r/aFlps9iUcD42vN4w@hovoldconsulting.com
> > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
> > ---
> >  drivers/firmware/efi/vars.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c
> > index 3700e98697676d8e6f04f061f447391503f9abba..11c5f785c09364f61642d82416822cb2e1a027fd 100644
> > --- a/drivers/firmware/efi/vars.c
> > +++ b/drivers/firmware/efi/vars.c
> > @@ -227,6 +227,8 @@ efi_status_t efivar_set_variable_locked(efi_char16_t *name, efi_guid_t *vendor,
> >  	setvar = __efivars->ops->set_variable_nonblocking;
> >  	if (!setvar || !nonblocking)
> >  		 setvar = __efivars->ops->set_variable;
> > +	if (!setvar)
> > +		return EFI_WRITE_PROTECTED;
> >  
> >  	return setvar(name, vendor, attr, data_size, data);
> >  }
> 
> Johan

-- 
With best wishes
Dmitry