From patchwork Mon Mar 25 22:09:52 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Matthew Garrett X-Patchwork-Id: 10870191 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 458F614DE for ; Mon, 25 Mar 2019 22:11:07 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 2EDF728C1D for ; Mon, 25 Mar 2019 22:11:07 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 22F1C29067; Mon, 25 Mar 2019 22:11:07 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-14.5 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,RCVD_IN_DNSWL_HI,USER_IN_DEF_DKIM_WL autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id E19382905C for ; Mon, 25 Mar 2019 22:11:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731230AbfCYWLF (ORCPT ); Mon, 25 Mar 2019 18:11:05 -0400 Received: from mail-oi1-f201.google.com ([209.85.167.201]:53825 "EHLO mail-oi1-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731222AbfCYWLE (ORCPT ); Mon, 25 Mar 2019 18:11:04 -0400 Received: by mail-oi1-f201.google.com with SMTP id c21so4456522oig.20 for ; Mon, 25 Mar 2019 15:11:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=x7CSy5VUNpZirrVzUJKZsqK1/aML0MIeKkdBP16YKzM=; b=R7Xx65wIL06Y8y0QtQ8SG/vdw9oErMNuqyNB6rpB5YofGBaRNwZYi9l8tvDEOHq//V /7x3/yXesn18u/OkVNRZuU7tStW5UNqlnHJHIhVOhAP48zU7gjZ8VTuEGqyAyFngkllF USRMXhNXMQT/xjdXcye+DsqYj++Vp8TY8OnfRVUJHKTEzCLZEAl5bItoDD86rUuZknH3 TMbWURksHGo9J7sZ+klskcUUP10Gm6KYDacPW4+2Wz85Kl4XvtXO49dphZRi1jJlKepQ gvB4JPRZ131v68n3RPdYm6VB8q3LYfoYDbhCcBR8jDlY8zNRQzk8JLG0G5fXKDc0fzBe YB/A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=x7CSy5VUNpZirrVzUJKZsqK1/aML0MIeKkdBP16YKzM=; b=RcN3u4jHowhoycWeeLtQk2AQYRQ4izqonneM5oDlDv0Ltz+UnJhYLoXmw/0xtS09ZS 2IDucqdIeL5f3BZ2R36jFjiLbutWi6aGg/Dyatc3N/7/jqVuGHItklLjQiwwQTo7orG/ xnCmSNtPRp8gRX5PaHvmxY6wzdgp7GAhDceibtXE2Nq6T6j2qlmYrTOWJq21OKwPxeRI KkR6L+PEalARjYvKSNFhHU6Ek5mqkjlvt+EnDRZtTp0IGBAuI06d6f9A/dl8LEKwPVbu 5hVFnUf0xoq8rtWwRjEWx13bbATUOE0XCIUN3hoOOVCJNuFYZEqklu0jpEMKcgnM9FlQ StTw== X-Gm-Message-State: APjAAAVA4pgY4M+/hgtntWWy/nkcETs98HyK/TQKBW1AL6QWFNwSvTgb /reRlRYPnaF81lXUxaDBTPtwJFnVJ/2R5Zqf4kWy9g== X-Google-Smtp-Source: APXvYqxbvkDBtdhhh8lD62vZuxS8kradSSiukiXkadLNavl6KpgmoLZs1rLXZlzlDCrboZhOM501EYIRM+kDmvkrCLFvhQ== X-Received: by 2002:aca:b589:: with SMTP id e131mr13718240oif.91.1553551863717; Mon, 25 Mar 2019 15:11:03 -0700 (PDT) Date: Mon, 25 Mar 2019 15:09:52 -0700 In-Reply-To: <20190325220954.29054-1-matthewgarrett@google.com> Message-Id: <20190325220954.29054-26-matthewgarrett@google.com> Mime-Version: 1.0 References: <20190325220954.29054-1-matthewgarrett@google.com> X-Mailer: git-send-email 2.21.0.392.gf8f6787159e-goog Subject: [PATCH 25/27] debugfs: Restrict debugfs when the kernel is locked down From: Matthew Garrett To: jmorris@namei.org Cc: linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, dhowells@redhat.com, Andy Shevchenko , acpi4asus-user@lists.sourceforge.net, platform-driver-x86@vger.kernel.org, Matthew Garrett , Thomas Gleixner , Greg Kroah-Hartman , Matthew Garrett Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: X-Virus-Scanned: ClamAV using ClamSMTP From: David Howells Disallow opening of debugfs files that might be used to muck around when the kernel is locked down as various drivers give raw access to hardware through debugfs. Given the effort of auditing all 2000 or so files and manually fixing each one as necessary, I've chosen to apply a heuristic instead. The following changes are made: (1) chmod and chown are disallowed on debugfs objects (though the root dir can be modified by mount and remount, but I'm not worried about that). (2) When the kernel is locked down, only files with the following criteria are permitted to be opened: - The file must have mode 00444 - The file must not have ioctl methods - The file must not have mmap (3) When the kernel is locked down, files may only be opened for reading. Normal device interaction should be done through configfs, sysfs or a miscdev, not debugfs. Note that this makes it unnecessary to specifically lock down show_dsts(), show_devs() and show_call() in the asus-wmi driver. I would actually prefer to lock down all files by default and have the the files unlocked by the creator. This is tricky to manage correctly, though, as there are 19 creation functions and ~1600 call sites (some of them in loops scanning tables). Signed-off-by: David Howells cc: Andy Shevchenko cc: acpi4asus-user@lists.sourceforge.net cc: platform-driver-x86@vger.kernel.org cc: Matthew Garrett cc: Thomas Gleixner Cc: Greg Kroah-Hartman Signed-off-by: Matthew Garrett --- fs/debugfs/file.c | 28 ++++++++++++++++++++++++++++ fs/debugfs/inode.c | 30 ++++++++++++++++++++++++++++-- 2 files changed, 56 insertions(+), 2 deletions(-) diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c index 4fce1da7db23..c33042c1eff3 100644 --- a/fs/debugfs/file.c +++ b/fs/debugfs/file.c @@ -136,6 +136,25 @@ void debugfs_file_put(struct dentry *dentry) } EXPORT_SYMBOL_GPL(debugfs_file_put); +/* + * Only permit access to world-readable files when the kernel is locked down. + * We also need to exclude any file that has ways to write or alter it as root + * can bypass the permissions check. + */ +static bool debugfs_is_locked_down(struct inode *inode, + struct file *filp, + const struct file_operations *real_fops) +{ + if ((inode->i_mode & 07777) == 0444 && + !(filp->f_mode & FMODE_WRITE) && + !real_fops->unlocked_ioctl && + !real_fops->compat_ioctl && + !real_fops->mmap) + return false; + + return kernel_is_locked_down("debugfs"); +} + static int open_proxy_open(struct inode *inode, struct file *filp) { struct dentry *dentry = F_DENTRY(filp); @@ -147,6 +166,11 @@ static int open_proxy_open(struct inode *inode, struct file *filp) return r == -EIO ? -ENOENT : r; real_fops = debugfs_real_fops(filp); + + r = -EPERM; + if (debugfs_is_locked_down(inode, filp, real_fops)) + goto out; + real_fops = fops_get(real_fops); if (!real_fops) { /* Huh? Module did not clean up after itself at exit? */ @@ -272,6 +296,10 @@ static int full_proxy_open(struct inode *inode, struct file *filp) return r == -EIO ? -ENOENT : r; real_fops = debugfs_real_fops(filp); + r = -EPERM; + if (debugfs_is_locked_down(inode, filp, real_fops)) + goto out; + real_fops = fops_get(real_fops); if (!real_fops) { /* Huh? Module did not cleanup after itself at exit? */ diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c index 13b01351dd1c..4daec17b8215 100644 --- a/fs/debugfs/inode.c +++ b/fs/debugfs/inode.c @@ -32,6 +32,31 @@ static struct vfsmount *debugfs_mount; static int debugfs_mount_count; static bool debugfs_registered; +/* + * Don't allow access attributes to be changed whilst the kernel is locked down + * so that we can use the file mode as part of a heuristic to determine whether + * to lock down individual files. + */ +static int debugfs_setattr(struct dentry *dentry, struct iattr *ia) +{ + if ((ia->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) && + kernel_is_locked_down("debugfs")) + return -EPERM; + return simple_setattr(dentry, ia); +} + +static const struct inode_operations debugfs_file_inode_operations = { + .setattr = debugfs_setattr, +}; +static const struct inode_operations debugfs_dir_inode_operations = { + .lookup = simple_lookup, + .setattr = debugfs_setattr, +}; +static const struct inode_operations debugfs_symlink_inode_operations = { + .get_link = simple_get_link, + .setattr = debugfs_setattr, +}; + static struct inode *debugfs_get_inode(struct super_block *sb) { struct inode *inode = new_inode(sb); @@ -356,6 +381,7 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode, inode->i_mode = mode; inode->i_private = data; + inode->i_op = &debugfs_file_inode_operations; inode->i_fop = proxy_fops; dentry->d_fsdata = (void *)((unsigned long)real_fops | DEBUGFS_FSDATA_IS_REAL_FOPS_BIT); @@ -513,7 +539,7 @@ struct dentry *debugfs_create_dir(const char *name, struct dentry *parent) return failed_creating(dentry); inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO; - inode->i_op = &simple_dir_inode_operations; + inode->i_op = &debugfs_dir_inode_operations; inode->i_fop = &simple_dir_operations; /* directory inodes start off with i_nlink == 2 (for "." entry) */ @@ -608,7 +634,7 @@ struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent, return failed_creating(dentry); } inode->i_mode = S_IFLNK | S_IRWXUGO; - inode->i_op = &simple_symlink_inode_operations; + inode->i_op = &debugfs_symlink_inode_operations; inode->i_link = link; d_instantiate(dentry, inode); return end_creating(dentry);