linux-kernel - Re: [GIT PULL] x86/iopl changes for v5.5

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Message-ID: <20191126195046.GA28296@gmail.com>
Date:   Tue, 26 Nov 2019 20:50:46 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [GIT PULL] x86/iopl changes for v5.5


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, Nov 25, 2019 at 8:16 AM Ingo Molnar <mingo@...nel.org> wrote:
> >
> > This tree implements a nice simplification of the iopl and ioperm code
> > that Thomas Gleixner discovered: we can implement the IO privilege
> > features of the iopl system call by using the IO permission bitmap in
> > permissive mode, while trapping CLI/STI/POPF/PUSHF uses in user-space if
> > they change the interrupt flag.
> 
> I've pulled it.
> 
> But do we have a test for something like this:
> 
>    ioperm(.. limited set of ports..)
>    access that limited set.
> 
>    special_sequence() {
>        iopl(3);
>        access some extended set
>        iopl(0)
>    }
> 
>    go back to access the limited set again
> 
> because there's subtle interactions with people using *both* iopl()
> and ioperm() and switching between the two. Historically you could
> trivially do the above, because they are entirely independent
> operations. Does it still work?
> 
> Too busy/lazy to check myself.

Yes, I went through the code with such scenarios in mind and I believe it 
all works correctly: the two bitmaps are independent and the granular one 
is preserved across iopl() interactions. But to make sure I'll write a 
testcase as well.

In any case I agree that this kind of behavior is very much part of the 
ABI, so if it doesn't work like that we'll fix it. :-)

Thanks,

	Ingo