From mboxrd@z Thu Jan 1 00:00:00 1970 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 6E9E8C00140 for ; Thu, 11 Aug 2022 00:42:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232677AbiHKAmp (ORCPT ); Wed, 10 Aug 2022 20:42:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53758 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231434AbiHKAmo (ORCPT ); Wed, 10 Aug 2022 20:42:44 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 84D68844C6 for ; Wed, 10 Aug 2022 17:42:42 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 1357C61126 for ; Thu, 11 Aug 2022 00:42:42 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5C8F8C433C1; Thu, 11 Aug 2022 00:42:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1660178561; bh=1I+f9QWc6LzVBMRUz5sWBMoPslD77CpFojjhR3L5CRM=; h=Date:To:From:Subject:From; b=v868JZmqJDEmXYfFTkhs/LS6G4JH/JNFJbvODGW5S6XDYYrqMjLnGjH8nRaAcFXOV fmGjZTBNYvUa7QGHdMlnGyZstqE/birCbFff1Z11ETZlruF7k0Wqt4ymUJPCorfwdk O/ZlCc+qPZqKDTk3z53CSCu0RmpVdVTcs/fDOGpU= Date: Wed, 10 Aug 2022 17:42:40 -0700 To: mm-commits@vger.kernel.org, yi.zhang@huawei.com, viro@zeniv.linux.org.uk, vbabka@suse.cz, surenb@google.com, skhan@linuxfoundation.org, shuah@kernel.org, rppt@kernel.org, peterx@redhat.com, namit@vmware.com, mike.kravetz@oracle.com, mgorman@techsingularity.net, ldv@altlinux.org, jack@suse.cz, hughd@google.com, glebfm@altlinux.org, dave.hansen@linux.intel.com, corbet@lwn.net, axelrasmussen@google.com, akpm@linux-foundation.org From: Andrew Morton Subject: + userfaultfd-add-dev-userfaultfd-for-fine-grained-access-control.patch added to mm-unstable branch Message-Id: <20220811004241.5C8F8C433C1@smtp.kernel.org> Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org The patch titled Subject: userfaultfd: add /dev/userfaultfd for fine grained access control has been added to the -mm mm-unstable branch. Its filename is userfaultfd-add-dev-userfaultfd-for-fine-grained-access-control.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/userfaultfd-add-dev-userfaultfd-for-fine-grained-access-control.patch This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: Axel Rasmussen Subject: userfaultfd: add /dev/userfaultfd for fine grained access control Date: Mon, 8 Aug 2022 10:56:11 -0700 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. Link: https://lkml.kernel.org/r/20220808175614.3885028-3-axelrasmussen@google.com Signed-off-by: Axel Rasmussen Acked-by: Nadav Amit Acked-by: Peter Xu Cc: Al Viro Cc: Dave Hansen Cc: Dmitry V. Levin Cc: Gleb Fotengauer-Malinovskiy Cc: Hugh Dickins Cc: Jan Kara Cc: Jonathan Corbet Cc: Mel Gorman Cc: Mike Kravetz Cc: Mike Rapoport Cc: Shuah Khan Cc: Shuah Khan Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Zhang Yi Signed-off-by: Andrew Morton --- fs/userfaultfd.c | 73 ++++++++++++++++++++++------- include/uapi/linux/userfaultfd.h | 4 + 2 files changed, 61 insertions(+), 16 deletions(-) --- a/fs/userfaultfd.c~userfaultfd-add-dev-userfaultfd-for-fine-grained-access-control +++ a/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_fa 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(vo 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, --- a/include/uapi/linux/userfaultfd.h~userfaultfd-add-dev-userfaultfd-for-fine-grained-access-control +++ a/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 _ Patches currently in -mm which might be from axelrasmussen@google.com are selftests-vm-add-hugetlb_shared-userfaultfd-test-to-run_vmtestssh.patch userfaultfd-add-dev-userfaultfd-for-fine-grained-access-control.patch userfaultfd-selftests-modify-selftest-to-use-dev-userfaultfd.patch userfaultfd-update-documentation-to-describe-dev-userfaultfd.patch selftests-vm-add-dev-userfaultfd-test-cases-to-run_vmtestssh.patch