linux-kernel - Re: [PATCH v2 4/9] efi: pstore: Omit efivars caching EFI varstore access layer

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Message-ID: <202206211500.B4B9EEB8@keescook>
Date:   Tue, 21 Jun 2022 15:00:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Peter Jones <pjones@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2 4/9] efi: pstore: Omit efivars caching EFI varstore
 access layer

On Tue, Jun 21, 2022 at 11:33:29PM +0200, Ard Biesheuvel wrote:
> On Tue, 21 Jun 2022 at 23:21, Kees Cook <keescook@...omium.org> wrote:
> >
> > On Tue, Jun 21, 2022 at 11:12:17PM +0200, Ard Biesheuvel wrote:
> > > On Tue, 21 Jun 2022 at 23:00, Kees Cook <keescook@...omium.org> wrote:
> > > >
> > > > On Tue, Jun 21, 2022 at 05:36:18PM +0200, Ard Biesheuvel wrote:
> > > > > Avoid the efivars layer and simply call the newly introduced EFI
> > > > > varstore helpers instead. This simplifies the code substantially, and
> > > > > also allows us to remove some hacks in the shared efivars layer that
> > > > > were added for efi-pstore specifically.
> > > > >
> > > > > Since we don't store the name of the associated EFI variable into each
> > > > > pstore record when enumerating them, we have to guess the variable name
> > > > > it was constructed from at deletion time, since we no longer keep a
> > > > > shadow copy of the variable store. To make this a bit more exact, store
> > > > > the CRC-32 of the ASCII name into the pstore record's ECC region so we
> > > > > can use it later to make an educated guess regarding the name of the EFI
> > > > > variable.
> > > >
> > > > I wonder if pstore_record should have a "private" field for backends to
> > > > use? That seems like it solve the need for overloading the ecc field,
> > > > and allow for arbitrarily more information to be stored (i.e. store full
> > > > efi var name instead of an easily-colliding crc32?)
> > > >
> > >
> > > We could easily add that - we'd just have to decide how to free the
> > > memory it points to.
> >
> > I assume the pstore core could do that since it manages the record
> > lifetime already?
> >
> 
> So if priv is non-NULL when it frees the record, it passes it to kfree() ?

That's my idea, yeah. I *think* it'll work; I haven't taken a
super-close look, though.

-- 
Kees Cook