From patchwork Wed Aug 17 21:47:25 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12946510 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 6B239C25B08 for ; Wed, 17 Aug 2022 21:47:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242355AbiHQVrx (ORCPT ); Wed, 17 Aug 2022 17:47:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48952 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242045AbiHQVrj (ORCPT ); Wed, 17 Aug 2022 17:47:39 -0400 Received: from mail-yw1-x1149.google.com (mail-yw1-x1149.google.com [IPv6:2607:f8b0:4864:20::1149]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3353EA9241 for ; Wed, 17 Aug 2022 14:47:38 -0700 (PDT) Received: by mail-yw1-x1149.google.com with SMTP id 00721157ae682-334228502a8so81353427b3.20 for ; Wed, 17 Aug 2022 14:47:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:from:subject:references:mime-version:message-id:in-reply-to :date:from:to:cc; bh=yLHz07eKQlXccHA6tj5KlO991qOgQ0D3ylm0UoRMXpc=; b=AFkI/b8HdD4q3yRB5+26vHFE9o4APehNtEu0VteYUmrgnOysiE3ghPo13NauvFGU9n Zr4npnGPfTSuPwggNDIjAS1jpr7fXLGgPhN6rpXH0hR6az+rxAtX4665tCjq2Tll3Szj 5r2uXOvejkfBsUxd9KquMV6pNcQqlTmPKGJIhBSFekf5LE7mg1z6z6LMfYrkiA63SW65 QR/24mlh+d/YbrjknvYPY67p2rwdns6PHNRwVZGM6X7gOMsoeKkOBUOWNsF6LmpwW1Rd 59smdhXwVPyn1YCFGYo9dDW61ASNf2SU1kRSxBeBjGFKddtgXZzcK4rHcAeN+fcAO4l3 eUPg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:from:subject:references:mime-version:message-id:in-reply-to :date:x-gm-message-state:from:to:cc; bh=yLHz07eKQlXccHA6tj5KlO991qOgQ0D3ylm0UoRMXpc=; b=XaocHf51kvyRIaW+MxDujwSuZ1fPtmeaLTDtVTUQmEV5c6nJIz5O6mdwdulS5QaZue vm5Qx82QLBwSE6n6mCk8e7+T9G8qwOxgIuairhxPmakEZ2QAmAgNFj/kN3faS6tcd+eI tTAdDwa/3LPPpgP6m+JQ+eke0oEpPdrk5Sta7bjHXWfhI3+PYFCv0RF1NvQ/pqayIKVu eMzQiewVBnlXEFc0uxfYuKdkxWPZdUgb3WI+BzuBBjkJe/Ux+0LJDJQPqHKvXPvilbll TT+3/YoAF6hwQV8qcy6rzVmYlLy8BYNILVyw05mA2DmiS/I8y3foLjeVxu+mzOb/39Ts QuSA== X-Gm-Message-State: ACgBeo0FYHjmVRhT3fyTbwT6SVPq7U7W0dVElToimdnulmC7S8Zbg+AE pk67Xy0/NmDXKzEq+kfGfBX42oaZ4+6yaNK6KYMn X-Google-Smtp-Source: AA6agR7ePwk0i6PZYAKjGabK73VjHMKkbAObzZxln/nSCiLmaTdDEOnI/dPYoQiSFSwMRVhwJWrsB0QOC+QFM9QDU20V X-Received: from ajr0.svl.corp.google.com ([2620:15c:2d4:203:2f41:f176:4bac:b729]) (user=axelrasmussen job=sendgmr) by 2002:a05:6902:1206:b0:676:e465:24b1 with SMTP id s6-20020a056902120600b00676e46524b1mr204503ybu.323.1660772857476; Wed, 17 Aug 2022 14:47:37 -0700 (PDT) Date: Wed, 17 Aug 2022 14:47:25 -0700 In-Reply-To: <20220817214728.489904-1-axelrasmussen@google.com> Message-Id: <20220817214728.489904-3-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220817214728.489904-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.37.1.595.g718a3a8f04-goog Subject: [PATCH v6 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control From: Axel Rasmussen To: Alexander Viro , Andrew Morton , Dave Hansen , "Dmitry V . Levin" , Gleb Fotengauer-Malinovskiy , Hugh Dickins , Jan Kara , Jonathan Corbet , Mel Gorman , Mike Kravetz , Mike Rapoport , Nadav Amit , Peter Xu , Shuah Khan , Suren Baghdasaryan , Vlastimil Babka , zhangyi Cc: Axel Rasmussen , linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-security-module@vger.kernel.org, Mike Rapoport 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. To achieve this, add a /dev/userfaultfd misc device. This device provides an alternative to the userfaultfd(2) syscall for the creation of new userfaultfds. The idea is, any userfaultfds created this way will be able to handle kernel faults, without the caller having any special capabilities. Access to this mechanism is instead restricted using e.g. standard filesystem permissions. Acked-by: Mike Rapoport Acked-by: Nadav Amit Acked-by: Peter Xu Signed-off-by: Axel Rasmussen --- fs/userfaultfd.c | 73 +++++++++++++++++++++++++------- include/uapi/linux/userfaultfd.h | 4 ++ 2 files changed, 61 insertions(+), 16 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 1c44bf75f916..698e768d5c3d 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -30,6 +30,7 @@ #include #include #include +#include int sysctl_unprivileged_userfaultfd __read_mostly; @@ -415,13 +416,8 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) if (ctx->features & UFFD_FEATURE_SIGBUS) goto out; - if ((vmf->flags & FAULT_FLAG_USER) == 0 && - 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"); + if (!(vmf->flags & FAULT_FLAG_USER) && (ctx->flags & UFFD_USER_MODE_ONLY)) goto out; - } /* * If it's already released don't get it. This avoids to loop @@ -2052,20 +2048,11 @@ static void init_once_userfaultfd_ctx(void *mem) seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock); } -SYSCALL_DEFINE1(userfaultfd, int, flags) +static int new_userfaultfd(int flags) { struct userfaultfd_ctx *ctx; int fd; - if (!sysctl_unprivileged_userfaultfd && - (flags & UFFD_USER_MODE_ONLY) == 0 && - !capable(CAP_SYS_PTRACE)) { - 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"); - return -EPERM; - } - BUG_ON(!current->mm); /* Check the UFFD_* constants for consistency. */ @@ -2098,8 +2085,62 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) return fd; } +static inline bool userfaultfd_syscall_allowed(int flags) +{ + /* Userspace-only page faults are always allowed */ + if (flags & UFFD_USER_MODE_ONLY) + return true; + + /* + * The user is requesting a userfaultfd which can handle kernel faults. + * Privileged users are always allowed to do this. + */ + if (capable(CAP_SYS_PTRACE)) + return true; + + /* Otherwise, access to kernel fault handling is sysctl controlled. */ + return sysctl_unprivileged_userfaultfd; +} + +SYSCALL_DEFINE1(userfaultfd, int, flags) +{ + if (!userfaultfd_syscall_allowed(flags)) + return -EPERM; + + return new_userfaultfd(flags); +} + +static int userfaultfd_dev_open(struct inode *inode, struct file *file) +{ + return 0; +} + +static long userfaultfd_dev_ioctl(struct file *file, unsigned int cmd, unsigned long flags) +{ + if (cmd != USERFAULTFD_IOC_NEW) + return -EINVAL; + + return new_userfaultfd(flags); +} + +static const struct file_operations userfaultfd_dev_fops = { + .open = userfaultfd_dev_open, + .unlocked_ioctl = userfaultfd_dev_ioctl, + .compat_ioctl = userfaultfd_dev_ioctl, + .owner = THIS_MODULE, + .llseek = noop_llseek, +}; + +static struct miscdevice userfaultfd_misc = { + .minor = MISC_DYNAMIC_MINOR, + .name = "userfaultfd", + .fops = &userfaultfd_dev_fops +}; + static int __init userfaultfd_init(void) { + WARN_ON(misc_register(&userfaultfd_misc)); + userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache", sizeof(struct userfaultfd_ctx), 0, diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h index 7d32b1e797fb..005e5e306266 100644 --- a/include/uapi/linux/userfaultfd.h +++ b/include/uapi/linux/userfaultfd.h @@ -12,6 +12,10 @@ #include +/* ioctls for /dev/userfaultfd */ +#define USERFAULTFD_IOC 0xAA +#define USERFAULTFD_IOC_NEW _IO(USERFAULTFD_IOC, 0x00) + /* * If the UFFDIO_API is upgraded someday, the UFFDIO_UNREGISTER and * UFFDIO_WAKE ioctls should be defined as _IOW and not as _IOR. In