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

[Thomas Gleixner]

On Fri, Sep 17 2021 at 11:20, Peter Zijlstra wrote:
> Subject: x86/iopl: Fake iopl(3) CLI/STI usage
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 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@xxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>