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

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Date:   Tue, 21 Sep 2021 19:31:36 +0200
From:   Ondrej Zary <linux@...y.sk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
Subject: Re: [tip: x86/core] x86/iopl: Fake iopl(3) CLI/STI usage

On Tuesday 21 September 2021 14:00:59 Peter Zijlstra wrote:
> On Tue, Sep 21, 2021 at 01:09:26PM +0200, Ondrej Zary wrote:
> > On Tuesday 21 September 2021, tip-bot2 for Peter Zijlstra wrote:
> > > The following commit has been merged into the x86/core branch of tip:
> > > 
> > > Commit-ID:     32e1ae626f295152d1fc9a3375214133cbe62878
> > > Gitweb:        https://git.kernel.org/tip/32e1ae626f295152d1fc9a3375214133cbe62878
> > > Author:        Peter Zijlstra <peterz@...radead.org>
> > > AuthorDate:    Fri, 17 Sep 2021 11:20:04 +02:00
> > > Committer:     Peter Zijlstra <peterz@...radead.org>
> > > CommitterDate: Sat, 18 Sep 2021 12:18:32 +02:00
> > > 
> > > x86/iopl: Fake iopl(3) CLI/STI usage
> > > 
> > > 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>
> > > Link: https://lkml.kernel.org/r/20210918090641.GD5106@worktop.programming.kicks-ass.net
> > 
> > Could this be backported to 5.10 kernel so it can get into the recently released Debian 11?
> 
> Thomas also asked about a stable tag, so I'll rebase and force-push
> these commits and add:
> 
> Cc: stable@...nel.org # v5.5+
> 
> to it.
> 

Thank you very much for great work.

-- 
Ondrej Zary