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

[Dmitry Baryshkov]

On Fri, Jun 27, 2025 at 02:27:46PM +0200, Johan Hovold wrote:
> On Thu, Jun 26, 2025 at 03:54:11PM +0300, Dmitry Baryshkov wrote:
> > On Thu, 26 Jun 2025 at 15:51, Johan Hovold <johan@xxxxxxxxxx> wrote:
> > >
> > > 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")).
> > 
> > _documented_. I'll update documentation for efivar_set_variable() in
> > the next iteration and add a check to the RTC driver. However I still
> > think that this patch is valid.
> 
> Still depends on *how* we want to address this.

I'd prefer to address it in both places.

> > > > > 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.
> > 
> > Good.
> 
> Ok, so then there are no current drivers that will benefit from your
> change, but you may (or may not) need it if you enable RO efivars on
> this particular platform. That is, this patch is not actually fixing
> anything that is broken currently.

I'd leave that to a discretion of EFI / EFI vars maintainers. RTC driver
definitely is broken in its current state.

-- 
With best wishes
Dmitry