linux-kernel - Re: [PATCH] efi/x86: use naked RET on mixed mode call wrapper

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Date:   Mon, 18 Jul 2022 11:34:02 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Borislav Petkov <bp@...e.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        stable <stable@...r.kernel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
Subject: Re: [PATCH] efi/x86: use naked RET on mixed mode call wrapper

On Mon, Jul 18, 2022 at 9:28 AM Borislav Petkov <bp@...e.de> wrote:
>
> So I'm being told we need to untrain on return from EFI to protect the
> kernel from it.

Why would we have to protect the kernel from EFI?

If we can't trust EFI, then the machine is already compromised. We
just *called* an EFI routine, if EFI is untrusted, it did something
random.

I mean, it could have already done something bad at boot time when it
loaded the kernel.

So no, let's not "protect ourselves from EFI".

             Linus