From patchwork Thu Feb 24 18:19:53 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12759114 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9CB87C433F5 for ; Thu, 24 Feb 2022 18:20:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232671AbiBXSUl (ORCPT ); Thu, 24 Feb 2022 13:20:41 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42516 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232583AbiBXSUk (ORCPT ); Thu, 24 Feb 2022 13:20:40 -0500 Received: from mail-yb1-xb4a.google.com (mail-yb1-xb4a.google.com [IPv6:2607:f8b0:4864:20::b4a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 44D892556CF for ; Thu, 24 Feb 2022 10:20:10 -0800 (PST) Received: by mail-yb1-xb4a.google.com with SMTP id e129-20020a25d387000000b006245d830ca6so577241ybf.13 for ; Thu, 24 Feb 2022 10:20:10 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:message-id:mime-version:subject:from:to:cc; bh=Kr3msHxcEEuMu9ueVhf1C+yPycv5P/dhrWgKPHzNZeU=; b=fPfQHUeXB8dFNhcE0pSs43bdxCMaS/5S9/DzhMcZXeydaAkCzPRYjyJPHchp7jv7Fl YcI6FCU2KPWMX0IIyPh1fLisosge1/9iqrTFR4l3gwQI7GOxBAm65qyTsWZdNkjOK/NE k4jN0vbm5oQiFlMVAdO2u8bXj4BrV70nE30yhUXw+c2oICHiL9Bcbrbx9AJSeWZ1qVOx O0RxVsAvhoJ96n6AihRhyP/iKtz9DskulRg3Xx3PELkjLDNWIoeUowiVk8EiM0w4TMMA qq+Tzg42a1SC40LYdWFOJm2uDwMGriG9HWn5UcdO7D//xszsaVryahaBE1Q/mOIF7Jnw cvTA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=Kr3msHxcEEuMu9ueVhf1C+yPycv5P/dhrWgKPHzNZeU=; b=gDgqk/s6E0KWjT/TFoqw8Dkf6hwSSevhYONB6snhuqxroiGi6G5HClVjMYpmpVGoi4 Gdi9rpUgc0rU1S8FUEn2cNiuuTjYnAI0D5+MHrp9mta1Xsqz1MB0B8gUjdW1YyyFsSXx Qei1X1G1w+2Rl+Zwic/7Fq6MBMffQN2+JkvT4CmiA3WZ8Q5YG2fSWqmYpbZj7pIFFOs3 xUCekE0BrVjNM6qmjDquR2hcHKI2M/2jA7+J4MxOskI6H6VBB/7RX1WdphudNwXA4EDc r/4iqBl+6lju1CX1lRI4mX12sa1z/95nKW7eq5c291I9ok4NndZWzhvX6nuuPbUsBkzC 7SSQ== X-Gm-Message-State: AOAM533VvQc1VGIAcWcBbwP6uxx+Odxj69IlgMHNcT13dpjTynTynSwL wjqwgmb+FeHREZq/iKCJBeXe+QFLg65qN5i4PWOc X-Google-Smtp-Source: ABdhPJykrFAu6LDou6qQ+GvUENavEY0K5NwYoXaucR3RWYojp3M1TWx2uHE2Wm2gvvSBpVRlJMazTjkK9Vnl7gjXfQT4 X-Received: from ajr0.svl.corp.google.com ([2620:15c:2cd:203:5d35:89ab:52c:6dea]) (user=axelrasmussen job=sendgmr) by 2002:a25:8887:0:b0:622:77:ecad with SMTP id d7-20020a258887000000b006220077ecadmr3666746ybl.30.1645726809505; Thu, 24 Feb 2022 10:20:09 -0800 (PST) Date: Thu, 24 Feb 2022 10:19:53 -0800 Message-Id: <20220224181953.1030665-1-axelrasmussen@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.35.1.574.g5d30c73bfb-goog Subject: [PATCH] userfaultfd, capability: introduce CAP_USERFAULTFD From: Axel Rasmussen To: Peter Xu , Andrea Arcangeli , Alexander Viro , Serge Hallyn , Paul Moore , Stephen Smalley , Eric Paris , Ondrej Mosnacek , "David S . Miller" , Jeremy Kerr Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, Axel Rasmussen Precedence: bulk List-ID: Historically, it has been shown that intercepting kernel faults with userfaultfd (thereby forcing the kernel to wait for an arbitrary amount of time) can be exploited, or at least can make some kinds of exploits easier. So, in 37cd0575b8 "userfaultfd: add UFFD_USER_MODE_ONLY" we changed things so, in order for kernel faults to be handled by userfaultfd, either the process needs CAP_SYS_PTRACE, or this sysctl must be configured so that any unprivileged user can do it. In a typical implementation of a hypervisor with live migration (take QEMU/KVM as one such example), we do indeed need to be able to handle kernel faults. But, both options above are less than ideal: - Toggling the sysctl increases attack surface by allowing any unprivileged user to do it. - Granting the live migration process CAP_SYS_PTRACE gives it this ability, but *also* the ability to "observe and control the execution of another process [...], and examine and change [its] memory and registers" (from ptrace(2)). This isn't something we need or want to be able to do, so granting this permission violates the "principle of least privilege". This is all a long winded way to say: we want a more fine-grained way to grant access to userfaultfd, without granting other additional permissions at the same time. So, add CAP_USERFAULTFD, for this specific case. Setup a helper which accepts either CAP_USERFAULTFD, or for backward compatibility reasons (existing userspaces may depend on the old way of doing things), CAP_SYS_PTRACE. One special case is UFFD_FEATURE_EVENT_FORK: this is left requiring only CAP_SYS_PTRACE, since it is specifically about manipulating the memory of another (child) process, it sems like a better fit the way it is. To my knowledge, this isn't a feature required by typical live migration implementations, so this doesn't obviate the above. Signed-off-by: Axel Rasmussen --- fs/userfaultfd.c | 6 +++--- include/linux/capability.h | 5 +++++ include/uapi/linux/capability.h | 7 ++++++- security/selinux/include/classmap.h | 4 ++-- 4 files changed, 16 insertions(+), 6 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index e26b10132d47..1ec0d9b49a70 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -411,7 +411,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) ctx->flags & UFFD_USER_MODE_ONLY) { printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd " "sysctl knob to 1 if kernel faults must be handled " - "without obtaining CAP_SYS_PTRACE capability\n"); + "without obtaining CAP_USERFAULTFD capability\n"); goto out; } @@ -2068,10 +2068,10 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) if (!sysctl_unprivileged_userfaultfd && (flags & UFFD_USER_MODE_ONLY) == 0 && - !capable(CAP_SYS_PTRACE)) { + !userfaultfd_capable()) { printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd " "sysctl knob to 1 if kernel faults must be handled " - "without obtaining CAP_SYS_PTRACE capability\n"); + "without obtaining CAP_USERFAULTFD capability\n"); return -EPERM; } diff --git a/include/linux/capability.h b/include/linux/capability.h index 65efb74c3585..f1e7b3506432 100644 --- a/include/linux/capability.h +++ b/include/linux/capability.h @@ -270,6 +270,11 @@ static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns) ns_capable(ns, CAP_SYS_ADMIN); } +static inline bool userfaultfd_capable(void) +{ + return capable(CAP_USERFAULTFD) || capable(CAP_SYS_PTRACE); +} + /* audit system wants to get cap info from files as well */ int get_vfs_caps_from_disk(struct user_namespace *mnt_userns, const struct dentry *dentry, diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h index 463d1ba2232a..83a5d8601508 100644 --- a/include/uapi/linux/capability.h +++ b/include/uapi/linux/capability.h @@ -231,6 +231,7 @@ struct vfs_ns_cap_data { #define CAP_SYS_CHROOT 18 /* Allow ptrace() of any process */ +/* Allow everything under CAP_USERFAULTFD for backward compatibility */ #define CAP_SYS_PTRACE 19 @@ -417,7 +418,11 @@ struct vfs_ns_cap_data { #define CAP_CHECKPOINT_RESTORE 40 -#define CAP_LAST_CAP CAP_CHECKPOINT_RESTORE +/* Allow intercepting kernel faults with userfaultfd */ + +#define CAP_USERFAULTFD 41 + +#define CAP_LAST_CAP CAP_USERFAULTFD #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP) diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h index 35aac62a662e..98e37b220159 100644 --- a/security/selinux/include/classmap.h +++ b/security/selinux/include/classmap.h @@ -28,9 +28,9 @@ #define COMMON_CAP2_PERMS "mac_override", "mac_admin", "syslog", \ "wake_alarm", "block_suspend", "audit_read", "perfmon", "bpf", \ - "checkpoint_restore" + "checkpoint_restore", "userfaultfd" -#if CAP_LAST_CAP > CAP_CHECKPOINT_RESTORE +#if CAP_LAST_CAP > CAP_USERFAULTFD #error New capability defined, please update COMMON_CAP2_PERMS. #endif