From: Andy Lutomirski <luto@amacapital.net>
To: Sasha Levin <sashal@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>,
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>
Subject: Re: [PATCH v9 00/17] Enable FSGSBASE instructions
Date: Wed, 22 Apr 2020 16:00:16 -0700 [thread overview]
Message-ID: <CALCETrUGuvMgvcsUWsdmUr-=6c6BpRpOKC1MN+E16g17U7vyMQ@mail.gmail.com> (raw)
In-Reply-To: <20200421205119.GK1809@sasha-vm>
On Tue, Apr 21, 2020 at 1:51 PM Sasha Levin <sashal@kernel.org> wrote:
>
> On Tue, Apr 21, 2020 at 01:21:39PM -0700, Andy Lutomirski wrote:
> >
> >
> >> On Apr 21, 2020, at 12:56 PM, Andi Kleen <ak@linux.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.
>
> That's what I referred to with "poke at it". While the behavior may be
> different, I fail to find anyone who cares.
>
> >>
> >> [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?
>
> Come on, how does one respond to this?
>
> Is there a real use case affected by this? If so, point it out and I'll
> be happy to go test it. This was already done (per your previous
> request) for gdb and rr.
>
gdb and rr are certainly a good start. If patches show up, I'll take a look.
next prev parent reply other threads:[~2020-04-22 23:00 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-04 18:15 [PATCH v9 00/17] Enable FSGSBASE instructions Chang S. Bae
2019-10-04 18:15 ` [PATCH v9 01/17] x86/ptrace: Prevent ptrace from clearing the FS/GS selector Chang S. Bae
2019-10-04 18:15 ` [PATCH v9 02/17] selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write Chang S. Bae
2019-10-04 18:15 ` [PATCH v9 03/17] x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2019-10-04 18:15 ` [PATCH v9 04/17] x86/entry/64: Clean up paranoid exit Chang S. Bae
2019-10-04 18:15 ` [PATCH v9 05/17] x86/entry/64: Switch CR3 before SWAPGS in paranoid entry Chang S. Bae
2019-10-04 18:15 ` [PATCH v9 06/17] x86/entry/64: Introduce the FIND_PERCPU_BASE macro Chang S. Bae
2019-10-04 18:15 ` [PATCH v9 07/17] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 08/17] x86/entry/64: Document GSBASE handling in the paranoid path Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 09/17] x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 10/17] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 11/17] x86/fsgsbase/64: Use FSGSBASE in switch_to() if available Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 12/17] x86/fsgsbase/64: Use FSGSBASE instructions on thread copy and ptrace Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 13/17] x86/speculation/swapgs: Check FSGSBASE in enabling SWAPGS mitigation Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 14/17] selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 15/17] x86/fsgsbase/64: Enable FSGSBASE on 64bit by default and add a chicken bit Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 16/17] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Chang S. Bae
2019-10-04 18:16 ` [PATCH v9 17/17] Documentation/x86/64: Add documentation for GS/FS addressing mode Chang S. Bae
2019-10-04 22:54 ` Randy Dunlap
2019-11-15 18:29 ` [PATCH v9 00/17] Enable FSGSBASE instructions Thomas Gleixner
2019-11-15 19:12 ` Andi Kleen
2019-11-29 14:56 ` Metzger, Markus T
2019-11-29 16:51 ` Andy Lutomirski
2019-12-02 8:23 ` Metzger, Markus T
2019-12-04 20:20 ` Andy Lutomirski
2019-12-10 8:27 ` Metzger, Markus T
2020-02-24 18:02 ` Bae, Chang Seok
2020-04-13 20:03 ` Sasha Levin
2020-04-14 0:32 ` Andi Kleen
2020-04-17 13:30 ` Sasha Levin
2020-04-17 15:52 ` Andy Lutomirski
2020-04-20 14:13 ` Andi Kleen
2020-04-20 17:14 ` Thomas Gleixner
2020-04-21 16:06 ` Sasha Levin
2020-04-21 16:49 ` Andy Lutomirski
2020-04-21 20:02 ` Andi Kleen
2020-04-21 17:15 ` Bae, Chang Seok
2020-04-21 19:56 ` Andi Kleen
2020-04-21 20:21 ` Andy Lutomirski
2020-04-21 20:51 ` Sasha Levin
2020-04-22 23:00 ` Andy Lutomirski [this message]
2020-04-23 4:08 ` Sasha Levin
2020-04-25 22:39 ` Thomas Gleixner
2020-04-26 2:52 ` Sasha Levin
2020-04-26 10:04 ` Thomas Gleixner
2020-04-14 15:47 ` Bae, Chang Seok
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