Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   "Yao, Yuan" <yuan.yao@intel.com>
To:     Dave Hansen <dave.hansen@linux.intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: RE: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for
 init_fpstate
Thread-Topic: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for
 init_fpstate
Thread-Index: AQHY3cA8G+rUHXTHGkezKzq8f0LSDa4Li1jQ
Date:   Thu, 13 Oct 2022 01:33:11 +0000
Message-ID: <BYAPR11MB3717FC0492B36C16AAB95F2095259@BYAPR11MB3717.namprd11.prod.outlook.com>
References: <20221011222425.866137-1-dave.hansen@linux.intel.com>
In-Reply-To: <20221011222425.866137-1-dave.hansen@linux.intel.com>
Accept-Language: en-US
Content-Language: en-US
dlp-version: 11.6.500.17
dlp-product: dlpe-windows
dlp-reaction: no-action
x-ms-exchange-antispam-messagedata-chunkcount: 1
x-ms-exchange-antispam-messagedata-0: =?us-ascii?Q?EUt1C6tSvUWM0S/PMoPsDQ8pNzaCNaTOC7gKosm0fApKuuf8/OAtY1XpO3G/?=
 =?us-ascii?Q?UHTYLq/R6rEQITyNGbDY2AF/ZenpY8V5dKM98c0vm761mEvoPEzHNbRNi5t1?=
 =?us-ascii?Q?xwM+FZY3huBjs8585jqc8xJLLZy7sJEhmLoWHJTv2Cw0kY7FLtfF38+HJVYg?=
 =?us-ascii?Q?8/lFNx1qSRV0UNTUj/mH8qE3F8eAKP9sOa8Vug5EvEc230A2kwTIBt3zptWY?=
 =?us-ascii?Q?tZOFKw8QzPjoHMdlM/G9dJPFp6Scsfv0MtaEDl/p8Xaag0ETgWCiXZKEN1JH?=
 =?us-ascii?Q?LN2awLbH/mZDKyxGTUjEo1SFI++ToazaIRUw1TPAiz6kApKCApnAFZQwusH9?=
 =?us-ascii?Q?t3khbMduSbDw4VtlZauryYDED7SiI6j/UisxbEIicVEmbSg/fIhkKrfcEndw?=
 =?us-ascii?Q?FMilM9l2uZ/4/oLKKZ4EJki2Zr5r4QyHhIt8a1XYOVOpZZAoI1yunnIxkCXx?=
 =?us-ascii?Q?7QNMGEB9FR5e7mDnLxIGjHoPlvkOKDmgduajsD2wqBzuumt4WZgBsBlWqPKA?=
 =?us-ascii?Q?q1HMueiuuYQ/XKvVKTkXTuJiGpgyaJkYx7QoVWC9L3KXpGx+ZwQKBNZqwsm8?=
 =?us-ascii?Q?AcMkd5BxZe3iVGH+MqdhtY0yOKN/4x6o74e6XbFpp7JfcJRLPYugwH3qht/p?=
 =?us-ascii?Q?89EAn38SmdDTHCl6UufjWNSxcttwHgPu7k4vy8XGsbB22lPeM+1xqG66FUuL?=
 =?us-ascii?Q?2AvUMoORadEi9fLFt9IgeNAULz23aGTOy6NZchEdu3WzTi16pz0t+CrppS7i?=
 =?us-ascii?Q?3NlneNepeD2VzG02guwyrQguKxoDpW2T0h8nk2m9dLc6/SPQVXIx8okoKOz4?=
 =?us-ascii?Q?ip3kNy31WeiZUBluPZO1Vs+OyEu+snRrY0I9F7y60nrixdAD/YnoDFcSJscu?=
 =?us-ascii?Q?4akolAr09w2x743UcA61iAweM4HcQSvOqYnm/FNEQ9aL3RQTqIfh9QAfDW5p?=
 =?us-ascii?Q?pOaE6RndpTTY99/b86CSQvVW2V9tuWc5GGGrVK3RkBeY4DN7QZmc9P1Z3tHh?=
 =?us-ascii?Q?IK5aCdaoDZVscifFEjzc8kEOnzkxvVKU2UPdH+yZv/JTFql2BpPSke5vjqvN?=
 =?us-ascii?Q?wMGbb4qRVTEvqOUMTVkC725lkqPCcwSLu4+fG+mOb9Vmuo7Fw45NTCfAcInc?=
 =?us-ascii?Q?Fw1JkfbQ+7G9IOepVqBuD1QqlFCRLVS7AT08UCOLWlHcclbjUuyXSH97B53a?=
 =?us-ascii?Q?dRIW3BgS9WfBeYD3iXcYTy+RnULyI+6YjWFISDhlTOzN7SgFLtnOvaGadKpb?=
 =?us-ascii?Q?Lq2ARJCFFNui8F+cl0hCkJITp11d0zVJ5BotStYFGrwvA4utFXSUrK+c8gx8?=
 =?us-ascii?Q?Ko5hUqS5Ne6hzsD8188C/Xmc1QbwWqCHeVsXJYEGmDpk72+WuIVMwX+Uj2lj?=
 =?us-ascii?Q?BzHPnyZZ2BYuBqTBOqPXmprFFF8daSi8YaWxw1bGYSaFdX7AecySZvDZU84/?=
 =?us-ascii?Q?50Ua7fYVYmKbueFsckdCszyytzLiW4ia5LtThxSeAPMcd9UTPF/1AbRUVopr?=
 =?us-ascii?Q?7x9wDCpn73ZeWz7jdlGAOTdXmkXA44WL/UrmookC47ssjBlmgKm3/PxpSvj6?=
 =?us-ascii?Q?AT4VQcgoCD0aAtf6BejBEw2sayVzepy8utczP0Of?=
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-AuthSource: BYAPR11MB3717.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-Network-Message-Id: d710bc3e-f781-4a87-5e29-08daacbae3ba
X-MS-Exchange-CrossTenant-originalarrivaltime: 13 Oct 2022 01:33:11.0726
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-mailboxtype: HOSTED
X-MS-Exchange-CrossTenant-userprincipalname: Cwz4UHYsRmzwC+Gsj9IjBYkF/cbddY5qYt+4LJX4BYlkgmIRtanMuyKejPXUz0sDWsrvlXgUNOKY7deEJaM41A==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: LV2PR11MB6047
X-OriginatorOrg: intel.com
X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,SPF_HELO_NONE,
        SPF_NONE autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

>-----Original Message-----
>From: Dave Hansen <dave.hansen@linux.intel.com>
>Sent: Wednesday, October 12, 2022 06:24
>To: linux-kernel@vger.kernel.org
>Cc: Bae, Chang Seok <chang.seok.bae@intel.com>; x86@kernel.org; Yao, Yuan =
<yuan.yao@intel.com>; Hansen, Dave
><dave.hansen@intel.com>; Thomas Gleixner <tglx@linutronix.de>
>Subject: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for init_f=
pstate
>
>From: Yuan Yao <yuan.yao@intel.com>
>
>This was found a couple of months ago in a big old AMX
>backport.  But, it looks to be a problem in mainline too.
>Please let me know if this looks OK.  I'd also especially
>appreciate some testing from folks that have AMX hardware
>handy.
>
>Builds and survives a quick boot test on non-AMX hardware.
>
>--
>
>=3D=3D Background =3D=3D
>
>'init_fpstate' is a sort of template for all of the fpstates
>that come after it.  It is copied when new processes are
>execve()'d or XRSTOR'd to get fpregs into a known state.
>
>That means that it represents the *starting* state for a
>process's fpstate which includes only the 'default' features.
>Dynamic features can, of course, be acquired later, but
>processes start with only default_features.
>
>During boot the kernel decides whether all fpstates will be
>kept in the compacted or uncompacted format.  This choice is
>communicated to the hardware via the XCOMP_BV field in all
>XSAVE buffers, including 'init_fpstate'.
>
>=3D=3D Problem =3D=3D
>
>But, the existing XCOMP_BV calculation is incorrect.  It uses
>the set of 'max_features', not the default features.
>
>As a result, when XRSTOR operates on buffers derived from
>'init_fpstate', it may attempt to "tickle" dynamic features which
>are at offsets for which there is no space allocated in the
>fpstate.
>
>=3D=3D Scope =3D=3D
>
>This normally results in a relatively harmless out-of-bounds
>memory read.  It's harmless because it never gets consumed.  But,
>if the fpstate is next to some unmapped memory, this "tickle" can
>cause a page fault and an oops.
>
>This only causes issues on systems when dynamic features are
>available and when an XSAVE buffer is next to uninitialized
>memory.  In other words, it only affects recent Intel server
>CPUs, and in relatively few memory locations.
>
>Those two things are why it took relatively long to catch this.
>
>=3D=3D Solution =3D=3D
>
>Use 'default_features' to establish the init_fpstate
>xcomp_bv value.  Reset individual fpstate xcomp_bv values
>when the rest of the fpstate is reset.
>
>[ dhansen: add reset code from tglx, rewrites
>	   commit message and comments ]

Thanks to Dave's help on this! The new comment message is more
clear and easier to understand then before.

>
>Fixes: 1c253ff2287f ("x86/fpu: Move xstate feature masks to fpu_*_cfg")
>Suggested-by: Dave Hansen <dave.hansen@intel.com>
>Suggested-by: Thomas Gleixner <tglx@linutronix.de>
>Signed-off-by: Yuan Yao <yuan.yao@intel.com>
>Cc: stable@vger.kernel.org
>---
> arch/x86/kernel/fpu/core.c   | 3 +++
> arch/x86/kernel/fpu/xstate.c | 7 ++++++-
> 2 files changed, 9 insertions(+), 1 deletion(-)
>
>diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
>index 3b28c5b25e12..4d64de34da12 100644
>--- a/arch/x86/kernel/fpu/core.c
>+++ b/arch/x86/kernel/fpu/core.c
>@@ -526,6 +526,9 @@ static void __fpstate_reset(struct fpstate *fpstate, u=
64 xfd)
> 	fpstate->xfeatures	=3D fpu_kernel_cfg.default_features;
> 	fpstate->user_xfeatures	=3D fpu_user_cfg.default_features;
> 	fpstate->xfd		=3D xfd;
>+
>+	/* Ensure that xcomp_bv matches ->xfeatures */
>+	xstate_init_xcomp_bv(&fpstate->regs.xsave, fpstate->xfeatures);
> }
>
> void fpstate_reset(struct fpu *fpu)
>diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
>index c8340156bfd2..f9f45610c72f 100644
>--- a/arch/x86/kernel/fpu/xstate.c
>+++ b/arch/x86/kernel/fpu/xstate.c
>@@ -360,7 +360,12 @@ static void __init setup_init_fpu_buf(void)
>
> 	print_xstate_features();
>
>-	xstate_init_xcomp_bv(&init_fpstate.regs.xsave, fpu_kernel_cfg.max_featur=
es);
>+	/*
>+	 * 'init_fpstate' is sized for the default feature
>+	 * set without any of the dynamic features.
>+	 */
>+	xstate_init_xcomp_bv(&init_fpstate.regs.xsave,
>+			     fpu_kernel_cfg.default_features);
>
> 	/*
> 	 * Init all the features state with header.xfeatures being 0x0
>--
>2.34.1