From patchwork Wed Jun 1 21:09:47 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12867289 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 F2E37CCA47F for ; Wed, 1 Jun 2022 21:10:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231312AbiFAVKZ (ORCPT ); Wed, 1 Jun 2022 17:10:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54524 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231286AbiFAVKC (ORCPT ); Wed, 1 Jun 2022 17:10:02 -0400 Received: from mail-yw1-x114a.google.com (mail-yw1-x114a.google.com [IPv6:2607:f8b0:4864:20::114a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 406EB1181D for ; Wed, 1 Jun 2022 14:10:00 -0700 (PDT) Received: by mail-yw1-x114a.google.com with SMTP id 00721157ae682-30c2f3431b4so25287297b3.21 for ; Wed, 01 Jun 2022 14:10:00 -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=901BpGHrP+WIymgStsREu89QnSrikR1eEvI9Dywbhsg=; b=BDJQcZ3k6VMLBGyDFzZskGuhBtnhdv42pQSckQsLOWCyVY7Xqf+2kT63J4F/d4bERE eydSs350ZLrk9nUqPl9uN8eOgJ01RknjccM4Di1YTbAf9nU4OlzFg7cxyxjYQ5EKFWXr hSQZYI/wunCARCD3fV8pt2KU6p/TGSGgBqHFrFpAyh3tcUXRhX73g6MT4b/9yRUbKN4d Yab5MzaGyeDAbdQugNMawgPRymIxc7tEcm+P8nS8wWhvmL/iUv/XqZ+RzapypeyousAZ T3evxkBkhGTICfx4jRI64IFNbHtW8RkuJv9zbUpafjqZyn8P8WygujdsYUelbZVUYBtU f4Zg== 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=901BpGHrP+WIymgStsREu89QnSrikR1eEvI9Dywbhsg=; b=CwS/TD6ydo1FHkBnQ3ilQPA0m79hS0CwZ8HkXwDWMdF1h7iofnVQ4qIEZcJt7CkKXp 6x8K11xyDH2Avic9u3x1khNlijizh0bo7RlWCAH2f3O9vhA6BICVc8iGWbm+Acq4mGKj q1l3Y7wE9Qa0fzmAzyupCb0mf/4+5zBdxpiPTqNWxTIQBLs6CBZHBfmtR1DxNMZMarDU D6NgFnotDac1SZLN4xIErOJtMVysuVaH76LoVU3fjUWLZYV3/hKBWuJPY2BaAwQHEXyU UtSQm5ewLHTwKrmRINKFUesvz2KiXbs+QcWX3nC9AwotMJCcb+D/NBtcn3uxqXT63hxL itmg== X-Gm-Message-State: AOAM531wiRjrK0hbFgtaiEQ8tp73lKsgayJm1l1CzCWgqYrbjewkD5ek FivTqnvODYvxB8xyAchBW87NTZaHPs6h2axp7w6c X-Google-Smtp-Source: ABdhPJw5CfsqBtuSRUvpx1MwggnCJckvoUqHXGCXU20XJb4bXPK6ug8yRK0rKKD/5bVEMCBJoJnLvnmpm2vCkd8dTOYp X-Received: from ajr0.svl.corp.google.com ([2620:15c:2cd:203:aaec:e358:9f0e:2b26]) (user=axelrasmussen job=sendgmr) by 2002:a0d:d003:0:b0:2eb:6441:d9a3 with SMTP id s3-20020a0dd003000000b002eb6441d9a3mr1669520ywd.273.1654117799431; Wed, 01 Jun 2022 14:09:59 -0700 (PDT) Date: Wed, 1 Jun 2022 14:09:47 -0700 In-Reply-To: <20220601210951.3916598-1-axelrasmussen@google.com> Message-Id: <20220601210951.3916598-3-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220601210951.3916598-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.36.1.255.ge46751e96f-goog Subject: [PATCH v3 2/6] userfaultfd: add /dev/userfaultfd for fine grained access control From: Axel Rasmussen To: Alexander Viro , Andrew Morton , Charan Teja Reddy , 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-fsdevel@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 | 76 ++++++++++++++++++++++++++------ include/uapi/linux/userfaultfd.h | 4 ++ 2 files changed, 66 insertions(+), 14 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index e943370107d0..8b92c1398169 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 +2048,33 @@ 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_allowed(bool is_syscall, int flags) +{ + bool kernel_faults = !(flags & UFFD_USER_MODE_ONLY); + bool allow_unprivileged = sysctl_unprivileged_userfaultfd; + + /* userfaultfd(2) access is controlled by sysctl + capability. */ + if (is_syscall && kernel_faults) { + if (!allow_unprivileged && !capable(CAP_SYS_PTRACE)) + return false; + } + + /* + * For /dev/userfaultfd, access is to be controlled using e.g. + * permissions on the device node. We assume this is correctly + * configured by userspace, so we simply allow access here. + */ + + return true; +} + +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 (!userfaultfd_allowed(is_syscall, flags)) return -EPERM; - } BUG_ON(!current->mm); @@ -2083,6 +2093,10 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) refcount_set(&ctx->refcount, 1); ctx->flags = flags; ctx->features = 0; + /* + * If UFFD_USER_MODE_ONLY is not set, then userfaultfd_allowed() above + * decided that kernel faults were allowed and should be handled. + */ ctx->released = false; atomic_set(&ctx->mmap_changing, 0); ctx->mm = current->mm; @@ -2098,8 +2112,42 @@ 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