linux-kernel - Re: [PATCH v9 00/17] Enable FSGSBASE instructions

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Date:   Tue, 21 Apr 2020 13:21:39 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Sasha Levin <sashal@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "hpa@...or.com" <hpa@...or.com>, "bp@...en8.de" <bp@...en8.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Pedro Alves <palves@...hat.com>,
        Simon Marchi <simark@...ark.ca>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 00/17] Enable FSGSBASE instructions



> On Apr 21, 2020, at 12:56 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> 
> 
>> 
>> Andi's point is that there is no known user it breaks, and the Intel
>> folks did some digging into potential users who might be affected by
>> this, including 'rr' brought up by Andy, and concluded that there won't
>> be breakage as a result of this patchset:
>> 
>>    https://mail.mozilla.org/pipermail/rr-dev/2018-March/000616.html
>> 
>> Sure, if you poke at it you could see a behavior change, but is there
>> an actual user that will be affected by it? I suspect not.
> 
> Actually we don't know of any behavior changes caused by the kernel
> with selectors.
> 
> The application can change itself of course, but only if it uses the 
> new instructions, which no current application does.

If you use ptrace to change the gs selector, the behavior is different on a patched kernel.

Again, I’m not saying that the change is problematic. But I will say that the fact that anyone involved in this series keeps ignoring this fact makes me quite uncomfortable with the patch set.

> 
> [This was different in the original patch kit long ago which could
> change behavior on context switch for programs with out of sync selectors,
> but this has been long fixed]

That’s the issue I was referring to.

> 
> A debugger can also change behavior, but we're not aware of any case
> that it would break.

How hard did you look?

> 
> For rr or criu we're also not aware of any case that could break.
> 
> I honestly don't know what else could be done in due diligence.
> 
> Also just to reiterate merging this would immediately shave off
> hundreds of cycles in most context switches.
> 
> -Andi