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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v9 00/17] Enable FSGSBASE instructions
Date:   Tue, 21 Apr 2020 13:21:39 -0700
Message-Id: <5957D3CC-B54B-4A1A-998A-0E6F58589CF4@amacapital.net>
References: <20200421195629.GE608746@tassilo.jf.intel.com>
Cc:     Sasha Levin <sashal@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        "Metzger, Markus T" <markus.t.metzger@intel.com>,
        "hpa@zytor.com" <hpa@zytor.com>, "bp@alien8.de" <bp@alien8.de>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "Luck, Tony" <tony.luck@intel.com>,
        Pedro Alves <palves@redhat.com>,
        Simon Marchi <simark@simark.ca>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
In-Reply-To: <20200421195629.GE608746@tassilo.jf.intel.com>
To:     Andi Kleen <ak@linux.intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 21, 2020, at 12:56 PM, Andi Kleen <ak@linux.intel.com> wrote:
>=20
> =EF=BB=BF
>>=20
>> 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:
>>=20
>>    https://mail.mozilla.org/pipermail/rr-dev/2018-March/000616.html
>>=20
>> 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.
>=20
> Actually we don't know of any behavior changes caused by the kernel
> with selectors.
>=20
> The application can change itself of course, but only if it uses the=20
> new instructions, which no current application does.

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

Again, I=E2=80=99m not saying that the change is problematic. But I will say=
 that the fact that anyone involved in this series keeps ignoring this fact m=
akes me quite uncomfortable with the patch set.

>=20
> [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=E2=80=99s the issue I was referring to.

>=20
> A debugger can also change behavior, but we're not aware of any case
> that it would break.

How hard did you look?

>=20
> For rr or criu we're also not aware of any case that could break.
>=20
> I honestly don't know what else could be done in due diligence.
>=20
> Also just to reiterate merging this would immediately shave off
> hundreds of cycles in most context switches.
>=20
> -Andi