From patchwork Fri Apr 22 21:29:41 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 12824077 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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8014BC433EF for ; Fri, 22 Apr 2022 21:29:57 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 10C876B0075; Fri, 22 Apr 2022 17:29:57 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0E3736B0078; Fri, 22 Apr 2022 17:29:57 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id EC6F26B007D; Fri, 22 Apr 2022 17:29:56 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.26]) by kanga.kvack.org (Postfix) with ESMTP id DEA786B0075 for ; Fri, 22 Apr 2022 17:29:56 -0400 (EDT) Received: from smtpin24.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id BD191252ED for ; Fri, 22 Apr 2022 21:29:56 +0000 (UTC) X-FDA: 79385807592.24.D645234 Received: from mail-yw1-f202.google.com (mail-yw1-f202.google.com [209.85.128.202]) by imf16.hostedemail.com (Postfix) with ESMTP id 6271018003B for ; Fri, 22 Apr 2022 21:29:54 +0000 (UTC) Received: by mail-yw1-f202.google.com with SMTP id 00721157ae682-2eb7d137101so81744567b3.12 for ; Fri, 22 Apr 2022 14:29:56 -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=ZKwYZFMlPmL6KMhQQz8m//PR2YxeSx2kvEK7QsVigJI=; b=csJYqjI7+4H3NJVJax9NGS3pNlt2RUYB3+3McYgDnf45op0d8iUq0yCVP4z+6JvUFf 2Cpb+TtSM43LXHqZExuKefroEz7gh03gyTWpMXuqRZSxtuP2YUeMnuRu69CM4h9NHqu1 Rt1m64uZho/RVKCFiWhSXEHProJQP+tXVbfJDgG/1mE9kiZDWXOLeAbrnJgOQ5uYKaBq Baf2Z8XNif2KvlAHnovD2kwKU8KrPFdCWw6E/ubPmigUIOAAwqklhC5yORD60kwFj8M7 dV8w1yeJbu4JN5R08puEouNqPqaZwWoAvF3Gdbt5JP6S2N5c4GIRczR2DvpbQeHh3uEK 7FCA== 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=ZKwYZFMlPmL6KMhQQz8m//PR2YxeSx2kvEK7QsVigJI=; b=djuNn2Q3vMPe/2U3/guAzgMsqyY2QAI1X+KOj20mE7JpnpBSZ98POqVy47BTYhFLEQ FIIBxpoXJAF0WycHxIovAV9ENj8BVPYAgAqUO67cG3pxmePD6iQe9J757RxLuDN+faqQ Rnzl3rWujrr3OLPTp7HamLWLJO7W35Cg5abvKTCrhPySuKOqFO3n81o38mIkOAG6PAaV MO2HsDbuv4npSdzCpNmbB59osCs2HGvkuwQYqCmPXKDFndBVVlY1Tg7RKPkwRC3HldJl dQjuIBswTPWpY7VewrBVO8dZZqo4A9Utszqq+yeXCC9fio+mIxeHC7WrQNpLyV03Lkyk LbmA== X-Gm-Message-State: AOAM5329DOurc4fehYtWxI/WuK1XkFY+qLlO8o+2CQZssx+vNMOnfunU PwAPTnO1mZzStAU1iBhR+RhkRqfh6zEG3LtdHHhd X-Google-Smtp-Source: ABdhPJwxCsTz9OxLMDOgPGwJ76n2p6bPU/yWKc6iqu2BypRG7Slj3og+uNJnovkB/PwFjYqyrErsxCwHfIK4qfTFl31r X-Received: from ajr0.svl.corp.google.com ([2620:15c:2cd:203:7ba6:20ac:a8f7:1dbd]) (user=axelrasmussen job=sendgmr) by 2002:a25:8382:0:b0:63d:6201:fa73 with SMTP id t2-20020a258382000000b0063d6201fa73mr6272316ybk.55.1650662995479; Fri, 22 Apr 2022 14:29:55 -0700 (PDT) Date: Fri, 22 Apr 2022 14:29:41 -0700 In-Reply-To: <20220422212945.2227722-1-axelrasmussen@google.com> Message-Id: <20220422212945.2227722-3-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220422212945.2227722-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.36.0.rc2.479.g8af0fa9b8e-goog Subject: [PATCH v2 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 Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=csJYqjI7; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf16.hostedemail.com: domain of 3Ux5jYg0KCDYSpWdjSkemkkWfYggYdW.Ugedafmp-eecnSUc.gjY@flex--axelrasmussen.bounces.google.com designates 209.85.128.202 as permitted sender) smtp.mailfrom=3Ux5jYg0KCDYSpWdjSkemkkWfYggYdW.Ugedafmp-eecnSUc.gjY@flex--axelrasmussen.bounces.google.com X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 6271018003B X-Rspam-User: X-Stat-Signature: enegndtgjfznwjhhjuentf148nu5ofws X-HE-Tag: 1650662994-546910 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org 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. Signed-off-by: Axel Rasmussen --- fs/userfaultfd.c | 79 ++++++++++++++++++++++++++------ include/uapi/linux/userfaultfd.h | 4 ++ 2 files changed, 69 insertions(+), 14 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index aa0c47cb0d16..16d7573ab41a 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -29,6 +29,7 @@ #include #include #include +#include int sysctl_unprivileged_userfaultfd __read_mostly; @@ -65,6 +66,8 @@ struct userfaultfd_ctx { unsigned int flags; /* features requested from the userspace */ unsigned int features; + /* whether or not to handle kernel faults */ + bool handle_kernel_faults; /* released */ bool released; /* memory mappings are changing because of non-cooperative event */ @@ -410,13 +413,8 @@ 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->handle_kernel_faults) goto out; - } /* * If it's already released don't get it. This avoids to loop @@ -2064,19 +2062,33 @@ 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); @@ -2095,6 +2107,11 @@ 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->handle_kernel_faults = !(flags & UFFD_USER_MODE_ONLY); ctx->released = false; atomic_set(&ctx->mmap_changing, 0); ctx->mm = current->mm; @@ -2110,8 +2127,42 @@ 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 = compat_ptr_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 ef739054cb1c..032a35b3bbd2 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 _IOWR(USERFAULTFD_IOC, 0x00, int) + /* * If the UFFDIO_API is upgraded someday, the UFFDIO_UNREGISTER and * UFFDIO_WAKE ioctls should be defined as _IOW and not as _IOR. In