From patchwork Tue Jul 19 19:56:25 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 591767 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 842C3C43334 for ; Tue, 19 Jul 2022 19:56:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238780AbiGST4x (ORCPT ); Tue, 19 Jul 2022 15:56:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44098 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234379AbiGST4k (ORCPT ); Tue, 19 Jul 2022 15:56:40 -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 72F08599D2 for ; Tue, 19 Jul 2022 12:56:38 -0700 (PDT) Received: by mail-yw1-x1149.google.com with SMTP id 00721157ae682-31e62bc916aso12556047b3.19 for ; Tue, 19 Jul 2022 12:56:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=qMVUvtohpl5u0YhYXflPGUyFWvhgp3J9BbhXpFzblMU=; b=lnfO9vGuCg2XCgU/FaH826cH1zjvOw9G/iNyzi1HOhL3DxRT5bJcHvrfvhI4t6URAW ZP8ma2CBwGO2AVLagAv1r0AO895+URwxT6XMHfplaCjyUO6lJkILUzedOiepPxy4Eahr EhH0JgOK+S8z069u+yUEl/l3mZrtwpKQA9Xw28TuBzAjavO2bWFh8TRuv1X2CVR90KpU qPBKThQYdZup0H8ibLQRBekD27EueLVSQBgOG0oQLbT0STjwrqunbK5D9oWZXtUEf2be s1WMV4AYiUbOkEVKGSa0BPiWU1Jl6E6ZbPremSWY888QlqwUWVrZ7Do8xcodXxNg2E9I fJJw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=qMVUvtohpl5u0YhYXflPGUyFWvhgp3J9BbhXpFzblMU=; b=f0NK7ewgkH6h8+T46EY/SjtM3oLz4vXhYYttl1W3M54pXbtaYfi3kO4cepu9RRp1mM AIhfkUpwb6LzQEskKq3hmYkvd33FT52SR1OGOKKXZClUqYnppHFW+akd8d43VO7VU1rF GVuVKZOd8PsCZrRwsN3hkDeuVaxEHrqXTxg+p4/TMBWhzQRUBJcb0Jug32zR+3wljSiJ 7bZ0UMnxXK8DTJelBhBZUSXwUg8/SZqvX7Rwm7Fcnh5pB45OCHI62y87Rww+bOK/OADx srA+6xHB+iSITPr8Gh6alDm4O+PSRsrukQyHve5glltRSsDqbL/p8a89L9/EAiEe2x8C YjSA== X-Gm-Message-State: AJIora8OyIcqSq2afuhEIGQpGilpKGLWOjKyKK3Lb93muvpF0tr12qKA RaPiIQiz/HaZeOER2mbYBA09qW7h0IxkWmbUSs2t X-Google-Smtp-Source: AGRyM1tJ/Uc5B/XJoSXLcnF76pAX+DiA/uqYaRjrJAYUu9yZiW0d+SAGBCOg8WS/GtxAUC/6LkRBA2YKMCzMouUel+Kd X-Received: from ajr0.svl.corp.google.com ([2620:15c:2d4:203:a065:9221:e40d:4fbe]) (user=axelrasmussen job=sendgmr) by 2002:a25:6ed5:0:b0:669:8b84:bb57 with SMTP id j204-20020a256ed5000000b006698b84bb57mr32393560ybc.227.1658260597485; Tue, 19 Jul 2022 12:56:37 -0700 (PDT) Date: Tue, 19 Jul 2022 12:56:25 -0700 In-Reply-To: <20220719195628.3415852-1-axelrasmussen@google.com> Message-Id: <20220719195628.3415852-3-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220719195628.3415852-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.37.0.170.g444d1eabd0-goog Subject: [PATCH v4 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-mm@kvack.org, linux-kselftest@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org 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. Signed-off-by: Axel Rasmussen --- fs/userfaultfd.c | 69 +++++++++++++++++++++++++------- include/uapi/linux/userfaultfd.h | 4 ++ 2 files changed, 59 insertions(+), 14 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index e943370107d0..968f2517a281 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -30,6 +30,7 @@ #include #include #include +#include int sysctl_unprivileged_userfaultfd __read_mostly; @@ -413,13 +414,8 @@ 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 @@ -2052,19 +2048,30 @@ 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 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; +} + +static int new_userfaultfd(bool is_syscall, 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"); + if (is_syscall && !userfaultfd_syscall_allowed(flags)) return -EPERM; - } BUG_ON(!current->mm); @@ -2098,8 +2105,42 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) return fd; } +SYSCALL_DEFINE1(userfaultfd, int, flags) +{ + return new_userfaultfd(true, 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(false, 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