linux-kernel - Re: [PATCH] x86/iopl: Fake iopl(3) CLI/STI usage

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Date:   Fri, 17 Sep 2021 12:40:43 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>, Ondrej Zary <linux@...y.sk>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/iopl: Fake iopl(3) CLI/STI usage

On Fri, Sep 17 2021 at 11:20, Peter Zijlstra wrote:
> Subject: x86/iopl: Fake iopl(3) CLI/STI usage
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Thu, 16 Sep 2021 23:05:09 +0200
>
> Since commit c8137ace5638 ("x86/iopl: Restrict iopl() permission
> scope") it's possible to emulate iopl(3) using ioperm(), except for
> the CLI/STI usage.
>
> Userspace CLI/STI usage is very dubious (read broken), since any
> exception taken during that window can lead to rescheduling anyway (or
> worse). The IOPL(2) manpage even states that usage of CLI/STI is highly
> discouraged and might even crash the system.
>
> Of course, that won't stop people and HP has the dubious honour of
> being the first vendor to be found using this in their hp-health
> package.
>
> In order to enable this 'software' to still 'work', have the #GP treat
> the CLI/STI instructions as NOPs when iopl(3). Warn the user that
> their program is doing dubious things.
>
> Fixes: a24ca9976843 ("x86/iopl: Remove legacy IOPL option")
> Reported-by: Ondrej Zary <linux@...y.sk>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>