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From: "Bae, Chang Seok" <chang.seok.bae@intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: "Metzger, Markus T" <markus.t.metzger@intel.com>,
	Andi Kleen <ak@linux.intel.com>, "hpa@zytor.com" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"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: Mon, 24 Feb 2020 18:02:17 +0000	[thread overview]
Message-ID: <29FD6626-4347-4B79-A027-52E44C7FDE55@intel.com> (raw)
In-Reply-To: <CALCETrWb9jvwOPuupet4n5=JytbS-x37bnn=THniv_d8cNvf_Q@mail.gmail.com>


> On Dec 4, 2019, at 12:20, Andy Lutomirski <luto@kernel.org> wrote:
> 
> I think it could make sense to add a whole new ptrace() command to
> tell the tracee to, in effect, MOV a specified value to a segment
> register.  This call would have the actual correct semantics in which
> it would return an error code if the specified value is invalid and
> would return 0 on success.  And then a second ptrace() call could be
> issued to read out FSBASE or GSBASE if needed.  Would this be useful?
> What gdb commands would invoke it?

We consider new commands to access GDT/LDT that hpa posted before [1] may be
helpful. If the kernel provides the interfaces to ptracer, we expect GDB for
both 32-/64-bits can make such changes for inferior calls:
(1) When FS/GS selector only updated,
	GDB used to write the selector value via SETREGS. Now it can read the
	base value from the new APIs and write the base also. This change does
	not harm today's kernel, and it retains the legacy behavior on
	FSGSBASE-enabled kernels in the future.
(2) When FS/GS base only updated,
(3) When both FS/GS selector and base updated,
	GDB has no change from what it used to do. The new FSGSBASE-enabled
	kernel improves the behavior by keeping the base regardless of a
	selector.

The proposed change in GDB would do an additional GETREGS for every SETREGS
to obtain the old value. Other ptrace-users may need a similar patch if
sensitive to the outcome from writing FS/GS selector, but last time when we
surveyed for other tools [2, 3], we didn't find the issue. We also didn't
find actual users who rely on legacy behavior in practice.

We'd like to hear a clear opinion of whether the GDB changes along with the
new ptrace APIs are necessary and sufficient as preparing the FSGSBASE
support in the kernel.

[1] https://lore.kernel.org/patchwork/cover/954471/
[2] https://mail.mozilla.org/pipermail/rr-dev/2018-March/000616.html
[3] https://lists.openvz.org/pipermail/criu/2018-March/040654.html

Thanks,
Chang

  parent reply	other threads:[~2020-02-24 18:02 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 [this message]
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
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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