linux-kernel - Re: [Patch v3 Part2 1/9] x86/microcode: Taint kernel only if microcode loading was successful

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Message-ID: <Y9perwfaheZqAJWl@zn.tnic>
Date:   Wed, 1 Feb 2023 13:44:31 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Ashok Raj <ashok.raj@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Stefan Talpalaru <stefantalpalaru@...oo.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Peter Zilstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Martin Pohlack <mpohlack@...zon.de>,
        "Li, Aubrey" <aubrey.li@...el.com>
Subject: Re: [Patch v3 Part2 1/9] x86/microcode: Taint kernel only if
 microcode loading was successful

On Tue, Jan 31, 2023 at 02:54:03PM -0800, Ashok Raj wrote:
> It's not an error, only when request_microcode() returns UCODE_ERROR, should
> it return -EINVAL,

So looking at ->request_microcode_fw(), it looks like we return
UCODE_ERROR when something with parsing the blob has gone wrong. So I
guess we can return something more fitting here to state that we failed
while parsing the microcode blob from userspace: it is corrupted,
truncated, what not.

Looking at the error codes, this:

#define ELIBBAD         80      /* Accessing a corrupted shared library */

seems fitting as it has "corrupted" blob in the definition. EBADF sounds
fitting too. In any case, it should be a distinct error value which
hints at what goes wrong.

> This shouldn't be noisy, but if you think this isn't needed, it can go
> away.

I think all this preemptive development - it might make sense so let's
do it - needs to stop. If there's an *actual* real use and need for it
sure, but let's issue a printk just because is not one of them.

> When it fails due to current_rev < min_rev, Isn't it good to add indication
> to user space that it didn't succeed? Thomas wanted these return codes, so
> someone scripting can get a status after an attempt to load.

Return codes: yes. Random, flaky, potentially overwritten in the dmesg
ring buffer error strings - nope. Soon someone will come along and say,
"hey, don't touch those printk formats - my tool parses them and it'll
break if you do." Yeah, right.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette