From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9690CC433E0 for ; Thu, 25 Jun 2020 05:51:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6F5DE20709 for ; Thu, 25 Jun 2020 05:51:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389906AbgFYFvm (ORCPT ); Thu, 25 Jun 2020 01:51:42 -0400 Received: from mga02.intel.com ([134.134.136.20]:64281 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389446AbgFYFvm (ORCPT ); Thu, 25 Jun 2020 01:51:42 -0400 IronPort-SDR: tOS59YQIKHu1ZXf7kZmh0DaskICZfGa+ztH6iJ6CDn/hrg/aHfiv+YA54jmlnci01mSufC80C6 ogCMcnaxoeUQ== X-IronPort-AV: E=McAfee;i="6000,8403,9662"; a="133193217" X-IronPort-AV: E=Sophos;i="5.75,278,1589266800"; d="scan'208";a="133193217" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Jun 2020 22:51:41 -0700 IronPort-SDR: QzPu4addwR0m89RYgPZYOfjXsxqL6Eg0hXaKoAPNKxE6SpIkx8opFaflwYi9NZk9pWrIVzAFNJ kERFhtj3LALg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.75,278,1589266800"; d="scan'208";a="293776524" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.152]) by orsmga002.jf.intel.com with ESMTP; 24 Jun 2020 22:51:41 -0700 Date: Wed, 24 Jun 2020 22:51:40 -0700 From: Sean Christopherson To: Borislav Petkov Cc: x86-ml , Linus Torvalds , lkml , Tony Luck Subject: Re: [PATCH -v2.1] x86/msr: Filter MSR writes Message-ID: <20200625055140.GA2141@linux.intel.com> References: <20200612105026.GA22660@zn.tnic> <20200613154844.GB584@zn.tnic> <20200615063837.GA14668@zn.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200615063837.GA14668@zn.tnic> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 15, 2020 at 08:38:37AM +0200, Borislav Petkov wrote: > Here's an improved v2 with sparse warnings fixed: > > --- > Disable writing to MSRs from userspace by default. Writes can still be > allowed by supplying the allow_writes=1 module parameter and the kernel > will be tainted so that it shows in oopses. > > Having unfettered access to all MSRs on a system is and has always been > a disaster waiting to happen. Think performance counter MSRs, MSRs with > sticky or locked bits, MSRs making major system changes like loading > microcode, MTRRs, PAT configuration, TSC counter, security mitigations > MSRs, you name it. > > This also destroys all the kernel's caching of MSR values for > performance, as the recent case with MSR_AMD64_LS_CFG showed. > > Another example is writing MSRs by mistake by simply typing the wrong > MSR address. System freezes have been experienced that way. > > In general, poking at MSRs under the kernel's feet is a bad bad idea. > > So disable poking directly at the MSRs by default. If userspace still > wants to do that, then proper interfaces should be defined which > are under the kernel's control and accesses to those MSRs can be > synchronized and sanitized properly. > > Changelog: > - taint before WRMSR, all > - make param 0600, Sean. > - do not deny but log writes by default, Linus. > > [ Fix sparse warnings ] > Reported-by: kernel test robot > Signed-off-by: Borislav Petkov A few non-functional nits below. Tested-by: Sean Christopherson > --- > arch/x86/kernel/msr.c | 69 +++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 69 insertions(+) > > diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c > index 1547be359d7f..576c43e39247 100644 > --- a/arch/x86/kernel/msr.c > +++ b/arch/x86/kernel/msr.c > @@ -42,6 +42,14 @@ > static struct class *msr_class; > static enum cpuhp_state cpuhp_msr_state; > > +enum allow_write_msrs { > + MSR_WRITES_ON, > + MSR_WRITES_OFF, > + MSR_WRITES_DEFAULT, > +}; > + > +static enum allow_write_msrs allow_writes = MSR_WRITES_DEFAULT; > + > static ssize_t msr_read(struct file *file, char __user *buf, > size_t count, loff_t *ppos) > { > @@ -70,6 +78,24 @@ static ssize_t msr_read(struct file *file, char __user *buf, > return bytes ? bytes : err; > } > > +static int filter_write(u32 reg) > +{ > + switch (allow_writes) { > + case MSR_WRITES_ON: return 0; break; > + case MSR_WRITES_OFF: return -EPERM; break; The breaks after the returns are unnecessary. > + default: break; > + } > + > + if (reg == MSR_IA32_ENERGY_PERF_BIAS) > + return 0; > + > + pr_err_ratelimited("Write to unrecognized MSR 0x%x by %s\n" > + "Please report to x86@kernel.org\n", > + reg, current->comm); Maybe s/unrecognized/unauthorized? Unrecognized implies the kernel doesn't know anything about the MSR being written, which may not hold true. > + return 0; > +} > + > static ssize_t msr_write(struct file *file, const char __user *buf, > size_t count, loff_t *ppos) > { > @@ -84,6 +110,10 @@ static ssize_t msr_write(struct file *file, const char __user *buf, > if (err) > return err; > > + err = filter_write(reg); > + if (err) > + return err; > + > if (count % 8) > return -EINVAL; /* Invalid chunk size */ > > @@ -92,9 +122,13 @@ static ssize_t msr_write(struct file *file, const char __user *buf, > err = -EFAULT; > break; > } > + > + add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK); > + > err = wrmsr_safe_on_cpu(cpu, reg, data[0], data[1]); > if (err) > break; > + Random leftover whitespace change. > tmp += 2; > bytes += 8; > }