From patchwork Mon Aug 8 17:56:11 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12938963 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 D0E69C3F6B0 for ; Mon, 8 Aug 2022 17:56:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244085AbiHHR4j (ORCPT ); Mon, 8 Aug 2022 13:56:39 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33498 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244115AbiHHR4g (ORCPT ); Mon, 8 Aug 2022 13:56:36 -0400 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 6484D17A93 for ; Mon, 8 Aug 2022 10:56:26 -0700 (PDT) Received: by mail-yb1-xb4a.google.com with SMTP id a11-20020a5b090b000000b00672acbe13c1so7942058ybq.2 for ; Mon, 08 Aug 2022 10:56:26 -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=CGty+tEr/tk0rC9BCM7jIIeUNQhRK73+QrnZH4Sx+Qo=; b=We9ZS1gYNHd0gn622wtkQZkkXLSQ6n5HOJ1HPvcMn6CJe9K8/pj4UiHWzPJVnC0K0B KB6L7cVoeu5N6NnW1F34khKpIDrw1mcbvnCwQbMdIk0/CLPLXewvvLqtlmc6IzCHpSE/ /wIo6gf7S1BI8GxOK3AM7Diz6O0X77W5udYiZtxB6yoW8H43tHL3OIbXi+xHUmk/fLJw wBHTxXcUwNocVssKW/evR2A/RbNnH8Az+963jczK31ttLsFTtJZWRHzPAO6FdIvQcfhM B97mTxBSxdDrDNhW6dbeGmTnrZ8CS4D+Zo2TwtyPdB/cB3Zf/o3lOB+e5SPgAgx8QKaR l8iQ== 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=CGty+tEr/tk0rC9BCM7jIIeUNQhRK73+QrnZH4Sx+Qo=; b=BVINzW9JzA5lZhRzKMv0t04ORJTkDNqiYjF+mNXqc9NKpyhxPzhGwaDSZZYh3pUeLm 0FRZZU5nrGTNnAE5Kb0202LpVxJrGUn4iI70RLFauy3ivDI3jqjD1zfWoIke7hLb6cgt zivwDiWifT+HVI/FBdW0H7ksSAkOV9pM4tYGgcuahxp2tzTND+eOyAuv2WFVcKr9Uwa2 nnHs+gKNxSTKRHhKmkUJjF8MnAAF0HefBf1TeEVRstDysTVV0kxqluYcjYNt1Cm8MSdX 9TRfbpDkGEnLUuLutar3qT1Dxo/c/xdS7jRe1WJ3/lcaGAUpXTxHsxYtwmEcFhaKdltd gscA== X-Gm-Message-State: ACgBeo1KBjBz94JoxP3lpG7AjIstgItBg/4dA/UA3YUBjWMx39EvrhnU WWWUSUd7zHQ4015T0GxC7ppkt3kIHhA2cXuae6Dv X-Google-Smtp-Source: AA6agR429+nQwLS+GTRt3TL8rFi5rkaCzsd+auyC+wUJV5p/vxBojaGA7Fdoexd8LBuTTZhZ7DViwZN1XsYfSYgYIEzl X-Received: from ajr0.svl.corp.google.com ([2620:15c:2d4:203:7a2a:3bb5:f3a0:3bbc]) (user=axelrasmussen job=sendgmr) by 2002:a81:7406:0:b0:322:64d1:3035 with SMTP id p6-20020a817406000000b0032264d13035mr19673303ywc.279.1659981385704; Mon, 08 Aug 2022 10:56:25 -0700 (PDT) Date: Mon, 8 Aug 2022 10:56:11 -0700 In-Reply-To: <20220808175614.3885028-1-axelrasmussen@google.com> Message-Id: <20220808175614.3885028-3-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220808175614.3885028-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.37.1.559.g78731f0fdb-goog Subject: [PATCH v5 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 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: Nadav Amit Acked-by: Peter Xu Signed-off-by: Axel Rasmussen Acked-by: Mike Rapoport --- 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