From patchwork Wed Aug 26 01:39:58 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Lokesh Gidra X-Patchwork-Id: 11737057 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 2CDF6722 for ; Wed, 26 Aug 2020 01:40:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 133DC2074A for ; Wed, 26 Aug 2020 01:40:22 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=google.com header.i=@google.com header.b="Gp9NVG0Z" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726779AbgHZBkR (ORCPT ); Tue, 25 Aug 2020 21:40:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52694 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726706AbgHZBkM (ORCPT ); Tue, 25 Aug 2020 21:40:12 -0400 Received: from mail-pg1-x54a.google.com (mail-pg1-x54a.google.com [IPv6:2607:f8b0:4864:20::54a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 44548C061574 for ; Tue, 25 Aug 2020 18:40:10 -0700 (PDT) Received: by mail-pg1-x54a.google.com with SMTP id k32so293883pgm.15 for ; Tue, 25 Aug 2020 18:40:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=sender:date:in-reply-to:message-id:mime-version:references:subject :from:to:cc; bh=QrfOzc6XPAwCgYDeycXOM2EpifoygU2U9TLCo43uwFY=; b=Gp9NVG0ZESXHmSZItMfVjNi/mKQmY5fuiZVCI7LgbDD0Pfi5ybSYImPwOJa25MEa0X ua6+a0hlj8fL33Ya6kESSfJaHhGxhu/4UzOsuKnHGDnPc0MvLqlfI0SMX5ojLWt7USFD EXz/vaGOAUXmVjNXZN3g8s1dfXUf6J3kKrRhURksX/sFRerGdp71sW+i0VzxsBf9T3sQ e1iV4RNOUXMnH+EWLrUSNuXtF+OVWHtPdBgqAHCoP4u3JT/21DOFHpVimdFLws/rA1m5 9+8F+8hdgjZzjRo4l3zAZpR6XbQWv5lixZubr53cBM9RaGqpjmL5m5Qmt10/Z87pyxQd RX+g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=QrfOzc6XPAwCgYDeycXOM2EpifoygU2U9TLCo43uwFY=; b=FQuVe7I9pWe+uP+oZ9onOX+o6VJoERmDH4fUyzOaxSwhttVR79LbDa+LFVs4QmxeDI KSQQWYub9dz+mCkQEc6Qw+Dy7KK0VMM1F9j66n7zVUP6T4rR7NINJ89DU6v+FDC1azl+ wTJBna4vS7mXNYW+xH27OR6RCdjk+gNJ5+OgfNvIXtQkqKQ0xfA6GqO2ix9d7JNf/HPm du9EKp5+flZ/yZLDxnCKl6cX/PzQpAy8/Wvxv0WWdllvpo27HAz+kmj9WqbuWp3DUhcF Tov4IiY3uqQQpr0mx0bQ627zJ1fbs6MKAyNzQGtaRiVw2e4NvmRrooVxn2a3nQuhjPc6 YH6w== X-Gm-Message-State: AOAM53306Lo7V/aoltElOLFjRydO+3PHKgmioxi4dIoyOnuzQQffmtk7 KQlWSM59GnU/jj44SF3qOxilEBzksspNyMQ/9w== X-Google-Smtp-Source: ABdhPJyuRHyDek0VQzUH9/QGNGLtchxKND8ZsBU0EPK7mBXtEMW1Vk/5gFl9Wx92oYp93X1Yy9DmZ2wtuCvjjxP7RQ== X-Received: from lg.mtv.corp.google.com ([2620:15c:211:202:f693:9fff:fef4:29dd]) (user=lokeshgidra job=sendgmr) by 2002:a17:90b:1b47:: with SMTP id nv7mr3677042pjb.128.1598406009623; Tue, 25 Aug 2020 18:40:09 -0700 (PDT) Date: Tue, 25 Aug 2020 18:39:58 -0700 In-Reply-To: <20200826013959.1981270-1-lokeshgidra@google.com> Message-Id: <20200826013959.1981270-2-lokeshgidra@google.com> Mime-Version: 1.0 References: <20200826013959.1981270-1-lokeshgidra@google.com> X-Mailer: git-send-email 2.28.0.297.g1956fa8f8d-goog Subject: [PATCH v3 1/2] Add UFFD_USER_MODE_ONLY From: Lokesh Gidra To: Kees Cook , Jonathan Corbet , Peter Xu , Andrea Arcangeli , Sebastian Andrzej Siewior , Andrew Morton Cc: Alexander Viro , Stephen Smalley , Eric Biggers , Lokesh Gidra , Daniel Colascione , "Joel Fernandes (Google)" , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kaleshsingh@google.com, calin@google.com, surenb@google.com, nnk@google.com, jeffv@google.com, kernel-team@android.com, Mike Rapoport , Shaohua Li , Jerome Glisse , Mauro Carvalho Chehab , Johannes Weiner , Mel Gorman , Nitin Gupta , Vlastimil Babka , Iurii Zaikin , Luis Chamberlain , Daniel Colascione Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org userfaultfd handles page faults from both user and kernel code. Add a new UFFD_USER_MODE_ONLY flag for userfaultfd(2) that makes the resulting userfaultfd object refuse to handle faults from kernel mode, treating these faults as if SIGBUS were always raised, causing the kernel code to fail with EFAULT. A future patch adds a knob allowing administrators to give some processes the ability to create userfaultfd file objects only if they pass UFFD_USER_MODE_ONLY, reducing the likelihood that these processes will exploit userfaultfd's ability to delay kernel page faults to open timing windows for future exploits. Signed-off-by: Daniel Colascione Signed-off-by: Lokesh Gidra --- fs/userfaultfd.c | 6 +++++- include/uapi/linux/userfaultfd.h | 9 +++++++++ 2 files changed, 14 insertions(+), 1 deletion(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 0e4a3837da52..3191434057f3 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -405,6 +405,9 @@ 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) + goto out; /* * If it's already released don't get it. This avoids to loop @@ -1975,10 +1978,11 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) BUG_ON(!current->mm); /* Check the UFFD_* constants for consistency. */ + BUILD_BUG_ON(UFFD_USER_MODE_ONLY & UFFD_SHARED_FCNTL_FLAGS); BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC); BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK); - if (flags & ~UFFD_SHARED_FCNTL_FLAGS) + if (flags & ~(UFFD_SHARED_FCNTL_FLAGS | UFFD_USER_MODE_ONLY)) return -EINVAL; ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL); diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h index e7e98bde221f..5f2d88212f7c 100644 --- a/include/uapi/linux/userfaultfd.h +++ b/include/uapi/linux/userfaultfd.h @@ -257,4 +257,13 @@ struct uffdio_writeprotect { __u64 mode; }; +/* + * Flags for the userfaultfd(2) system call itself. + */ + +/* + * Create a userfaultfd that can handle page faults only in user mode. + */ +#define UFFD_USER_MODE_ONLY 1 + #endif /* _LINUX_USERFAULTFD_H */ From patchwork Wed Aug 26 01:39:59 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Lokesh Gidra X-Patchwork-Id: 11737059 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id BAFE9722 for ; Wed, 26 Aug 2020 01:40:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 975782072D for ; Wed, 26 Aug 2020 01:40:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=google.com header.i=@google.com header.b="Q7e3Em/t" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726646AbgHZBkX (ORCPT ); Tue, 25 Aug 2020 21:40:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52714 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726777AbgHZBkP (ORCPT ); Tue, 25 Aug 2020 21:40:15 -0400 Received: from mail-yb1-xb4a.google.com (mail-yb1-xb4a.google.com [IPv6:2607:f8b0:4864:20::b4a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 86099C061786 for ; 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Tue, 25 Aug 2020 18:40:13 -0700 (PDT) Date: Tue, 25 Aug 2020 18:39:59 -0700 In-Reply-To: <20200826013959.1981270-1-lokeshgidra@google.com> Message-Id: <20200826013959.1981270-3-lokeshgidra@google.com> Mime-Version: 1.0 References: <20200826013959.1981270-1-lokeshgidra@google.com> X-Mailer: git-send-email 2.28.0.297.g1956fa8f8d-goog Subject: [PATCH v3 2/2] Add user-mode only option to unprivileged_userfaultfd sysctl knob From: Lokesh Gidra To: Kees Cook , Jonathan Corbet , Peter Xu , Andrea Arcangeli , Sebastian Andrzej Siewior , Andrew Morton Cc: Alexander Viro , Stephen Smalley , Eric Biggers , Lokesh Gidra , Daniel Colascione , "Joel Fernandes (Google)" , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kaleshsingh@google.com, calin@google.com, surenb@google.com, nnk@google.com, jeffv@google.com, kernel-team@android.com, Mike Rapoport , Shaohua Li , Jerome Glisse , Mauro Carvalho Chehab , Johannes Weiner , Mel Gorman , Nitin Gupta , Vlastimil Babka , Iurii Zaikin , Luis Chamberlain , Daniel Colascione Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org A third option is added to 'unprivileged_userfaultfd' sysctl knob. When the knob is set to 2, it allows unprivileged users to call userfaultfd, like when it is set to 1, but with the restriction that page faults from only user-mode can be handled. In this mode, an unprivileged user (without SYS_CAP_PTRACE capability) must pass UFFD_USER_MODE_ONLY to userfaultd or the API will fail with EPERM. This facility allows administrators to reduce the likelihood that an attacker with access to userfaultfd can delay faulting kernel code to widen timing windows for other exploits. Signed-off-by: Daniel Colascione Signed-off-by: Lokesh Gidra --- Documentation/admin-guide/sysctl/vm.rst | 10 +++++++--- fs/userfaultfd.c | 10 ++++++++-- kernel/sysctl.c | 2 +- 3 files changed, 16 insertions(+), 6 deletions(-) diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index 4b9d2e8e9142..23d6feb79f5c 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -872,9 +872,13 @@ unprivileged_userfaultfd ======================== This flag controls whether unprivileged users can use the userfaultfd -system calls. Set this to 1 to allow unprivileged users to use the -userfaultfd system calls, or set this to 0 to restrict userfaultfd to only -privileged users (with SYS_CAP_PTRACE capability). +system calls. Set this to 0 to restrict userfaultfd to only privileged +users (with SYS_CAP_PTRACE capability), set this to 1 to allow unprivileged +users to use the userfaultfd system calls, or set this to 2 to restrict +unprivileged users to handle page faults in user mode only. In the last case, +users without SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for +userfaultfd to succeed. Prohibiting use of userfaultfd for handling faults +from kernel mode may make certain vulnerabilities more difficult to exploit. The default value is 1. diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 3191434057f3..26cb87cf492d 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -1972,8 +1972,14 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) struct userfaultfd_ctx *ctx; int fd; - if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE)) - return -EPERM; + switch (sysctl_unprivileged_userfaultfd) { + case 2: + if (flags & UFFD_USER_MODE_ONLY) + break; + case 0: + if (!capable(CAP_SYS_PTRACE)) + return -EPERM; + } BUG_ON(!current->mm); diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 287862f91717..7e94215dfff5 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -3119,7 +3119,7 @@ static struct ctl_table vm_table[] = { .mode = 0644, .proc_handler = proc_dointvec_minmax, .extra1 = SYSCTL_ZERO, - .extra2 = SYSCTL_ONE, + .extra2 = &two, }, #endif { }