Re: [PATCH v4 1/8] efi: efivars: don't crash in efivar_set_variable{,_locked} in r/o case

[Johan Hovold]

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).

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?

> Reported-by: Johan Hovold <johan@xxxxxxxxxx>
> Closes: https://lore.kernel.org/r/aFlps9iUcD42vN4w@xxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxxxxxxxx>
> ---
>  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