From patchwork Tue Apr 12 20:29:41 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 561082 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 9819DC433EF for ; Tue, 12 Apr 2022 20:41:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230169AbiDLUnO (ORCPT ); Tue, 12 Apr 2022 16:43:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58138 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230235AbiDLUnB (ORCPT ); Tue, 12 Apr 2022 16:43:01 -0400 Received: from mail-yb1-f202.google.com (mail-yb1-f202.google.com [209.85.219.202]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 06576888C3 for ; Tue, 12 Apr 2022 13:37:44 -0700 (PDT) Received: by mail-yb1-f202.google.com with SMTP id b65-20020a25e444000000b0063dd00480f8so34762ybh.13 for ; Tue, 12 Apr 2022 13:37:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:message-id:mime-version:subject:from:to:cc; bh=WxuM56vp7eyey8vt1AZa+KldYkNrgkjPuC+4UbMDXgc=; b=rIM9EOW8GOu9cyGO8TXJ6h03VJygSCKXO8crDJqnskQH89h0ZkNfJ9QcigR3yBGwJg qIZIakU+ukGgbzbtBtrcJRatlgtr2WsuT1uhRYMkDZoB5wk0eninAo43ymkRPOkirNrq /816AqwD33qIzqfBV4wacqsGUAdSBGLoiJdn++k8j9aJAvNiMRne7dHD0R17EQukclon jCkxn4wbNTcNTRMRaVaA+0YjqUxB72PaFAAjiPWKXOSO3MiH1+AQx3jMXyfiGcttDc6b hy8JW5fCyciu4xUl/9HYYP0ybCeWnGKSjYBNlj56ErlhIeelassl5VxG2u3/aJIXAJ0J q/Ig== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=WxuM56vp7eyey8vt1AZa+KldYkNrgkjPuC+4UbMDXgc=; b=VxDECcf3PqZJ2cj4YS0kc3u5GJtYC+yRyS76YTNEL4bAkS57NNdZNyjv/LgEdI2MYG T0rI+a47Q1sWimWA5nzjXxtIuZxjLpoVTxv8MV6ZDSwjGsoN9z2r7DkYJJTbA+QCseZw p8U/vXloEl2lSTI8aUdpr17N6YPBLj2HR1OEoveqUvE58I81eHe6fxKGj40MtV2LDUCN Ws7HJAmIfekbX3n/xj060Yzin06GKbqeEFhzPLCe8BlI0BJL1DXrVbC+Y9s9pmjCoQcV v+In7M8rf7B2awu5Fk6bxlnn055QAMMQGOWvvIE2O9Zy7EIlXeHmGjalCcvJ7XH+O+Zj y8Pw== X-Gm-Message-State: AOAM533f7G8n/kBVvrszu+bdoj3Wj/vVvN0NYe5IAzNVWMUHYAla0W69 bsVAPM3UFThNmF1G+ByS/vbI54ltonNvN0HaEHaF X-Google-Smtp-Source: ABdhPJwN1rKyVp/tc0fMg/HjGUO+kQyygAHoYo6COvoV6GzliqlsiWADXi0U2ptDq7ByWupmrnrJxjn86JmOaUcWFqlS X-Received: from ajr0.svl.corp.google.com ([2620:15c:2cd:203:8927:f9ed:8b14:ddae]) (user=axelrasmussen job=sendgmr) by 2002:a25:9391:0:b0:641:3842:c5c4 with SMTP id a17-20020a259391000000b006413842c5c4mr11278242ybm.323.1649795388862; Tue, 12 Apr 2022 13:29:48 -0700 (PDT) Date: Tue, 12 Apr 2022 13:29:41 -0700 Message-Id: <20220412202942.386981-1-axelrasmussen@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.35.1.1178.g4f1659d476-goog Subject: [PATCH 1/2] userfaultfd: add /dev/userfaultfd for fine grained access control From: Axel Rasmussen To: Alexander Viro , Andrew Morton , "Dmitry V . Levin" , Gleb Fotengauer-Malinovskiy , Mike Kravetz , Nadav Amit , Peter Xu , Shuah Khan Cc: Axel Rasmussen , 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 | 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 From patchwork Tue Apr 12 20:29:42 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Axel Rasmussen X-Patchwork-Id: 560125 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 04537C433FE for ; Tue, 12 Apr 2022 23:28:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230377AbiDLXaZ (ORCPT ); Tue, 12 Apr 2022 19:30:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58114 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230264AbiDLX3b (ORCPT ); Tue, 12 Apr 2022 19:29:31 -0400 Received: from mail-oi1-x249.google.com (mail-oi1-x249.google.com [IPv6:2607:f8b0:4864:20::249]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 72E7516809D for ; Tue, 12 Apr 2022 15:30:44 -0700 (PDT) Received: by mail-oi1-x249.google.com with SMTP id k25-20020a544699000000b002fa69ba89b6so84971oic.19 for ; Tue, 12 Apr 2022 15:30:44 -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=u+9yLdrvqv+15A9PShNP0Bj2lgMYcE44VCQG0+LZtrY=; b=mwfr+1QL20PQIeOZQZw4zuZEGgBuBUOGgqdqpg6THGQZKaFw5Wj8PV8H59rdqaaBLU 96DEcj2/yCle2oJspe3XQ+wvE7SynQC2qq8T45bjBHdWr775iHRDb1Awf9JLNlMzGg0Z g9Vavh5XkBcZCDWPUCYqQaA2bJ40hvxQocKz/LgmqNRswWSq0TrGjsidj6Uh160Cxgg9 Br/LGeUfE2qBk04WfS9pWdcxkZ6ZfhQUGAQUPaDVZVTwx2AivA1G7qb2eWdfFOoTI0KO 79/zwiYoOluzurGB1K8giL4K789L/VwUjzCmIrbzk2s/dgPratqPJ30cmLHHGCauiOdw Ag0Q== 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=u+9yLdrvqv+15A9PShNP0Bj2lgMYcE44VCQG0+LZtrY=; b=Z4ZDXVCV4vvbd71yc0gCWstB6gu/k8Da12tAIFirgOVqLFvMRJNuqAWQ6msq3Rn0my 62fnIoAp8rMWiBB1ny4HMvUEkMEizy1amPU3i6/J/rPrHtByUDtsfubX37U+pGXgU38o /SsLEGhYacLbotqZUm7mQrD8Muypkb9oVAThgN05PO5qxmPEae9PHb4RN3NAGMgk9bGM ocgmeORTB3yU25tARbYYLHFCBEOjjl5dU2u8zZ+JNC7/mrpmQU10CNum891feCW9EKS0 qHXJdxztH9lROO+Y78mPg1NWp97orgJSwKCrOV7lb8C2NUFoikFAOQfOqDudUwB/qWB9 OyuQ== X-Gm-Message-State: AOAM533jYzQ/31PTrGwP3atdw/iLGxAbnnIS5EfMD59xanjSnhNZm5Zt 5wxm7g+Us3ddtwTho9qyNtAQa5LhVoMhNPKv3ILp X-Google-Smtp-Source: ABdhPJzT2Zt45zo3Wf9Ef/5d1NiGtDnKycptXlg/R76ipboiOT8KRuG6nigv1WkYm1tB4Mt/wit/6RMj+xbJNsBmESHI X-Received: from ajr0.svl.corp.google.com ([2620:15c:2cd:203:8927:f9ed:8b14:ddae]) (user=axelrasmussen job=sendgmr) by 2002:a17:90b:3c47:b0:1cb:8121:dcc8 with SMTP id pm7-20020a17090b3c4700b001cb8121dcc8mr6997487pjb.35.1649795391430; Tue, 12 Apr 2022 13:29:51 -0700 (PDT) Date: Tue, 12 Apr 2022 13:29:42 -0700 In-Reply-To: <20220412202942.386981-1-axelrasmussen@google.com> Message-Id: <20220412202942.386981-2-axelrasmussen@google.com> Mime-Version: 1.0 References: <20220412202942.386981-1-axelrasmussen@google.com> X-Mailer: git-send-email 2.35.1.1178.g4f1659d476-goog Subject: [PATCH 2/2] userfaultfd: selftests: modify selftest to use /dev/userfaultfd From: Axel Rasmussen To: Alexander Viro , Andrew Morton , "Dmitry V . Levin" , Gleb Fotengauer-Malinovskiy , Mike Kravetz , Nadav Amit , Peter Xu , Shuah Khan Cc: Axel Rasmussen , 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 Prefer this new interface, but if using it fails for any reason just fall back to using userfaultfd(2) as before. Signed-off-by: Axel Rasmussen --- tools/testing/selftests/vm/userfaultfd.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c index 92a4516f8f0d..a50c430f036c 100644 --- a/tools/testing/selftests/vm/userfaultfd.c +++ b/tools/testing/selftests/vm/userfaultfd.c @@ -383,13 +383,32 @@ static void assert_expected_ioctls_present(uint64_t mode, uint64_t ioctls) } } +static void __userfaultfd_open_dev(void) +{ + int fd; + + uffd = -1; + fd = open("/dev/userfaultfd", O_RDWR | O_CLOEXEC); + if (fd < 0) + return; + + uffd = ioctl(fd, USERFAULTFD_IOC_NEW, + O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY); + close(fd); +} + static void userfaultfd_open(uint64_t *features) { struct uffdio_api uffdio_api; - uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY); + __userfaultfd_open_dev(); + if (uffd < 0) { + printf("/dev/userfaultfd failed, fallback to userfaultfd(2)\n"); + uffd = syscall(__NR_userfaultfd, + O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY); + } if (uffd < 0) - err("userfaultfd syscall not available in this kernel"); + err("userfaultfd syscall failed"); uffd_flags = fcntl(uffd, F_GETFD, NULL); uffdio_api.api = UFFD_API;