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[209.132.180.67]) by mx.google.com with ESMTP id w18si2631135pgf.439.2019.03.06.16.01.18; Wed, 06 Mar 2019 16:01:33 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=broUSFPS; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726761AbfCGAAi (ORCPT + 99 others); Wed, 6 Mar 2019 19:00:38 -0500 Received: from mail-vs1-f74.google.com ([209.85.217.74]:38988 "EHLO mail-vs1-f74.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726738AbfCGAAh (ORCPT ); Wed, 6 Mar 2019 19:00:37 -0500 Received: by mail-vs1-f74.google.com with SMTP id s11so1516021vsp.6 for ; Wed, 06 Mar 2019 16:00:36 -0800 (PST) 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=PgWFbOvfoB6c+12vVAFa3Lz4ErLVnkwWwwAyfBQPr2w=; b=broUSFPSUir25lqoupd3OxHTv04+SWv+B4euIHyqYPNAv1jJ9BiFpknVyEGmdyRBKc t28Mt56SZPb+37huuxcCEBUnqsc58DABkCZeTZCrflnWKxFv7n33obb9y1+PxFpuXGz+ Nr1dIPhKUadyForwMY93MiZMCml5LL/kg4xS9rw7aolnCuVpval623I52ne3t9qqkWnU j2wMAsXvMY4XXuhTQlzD+GLO96oWQqAP+VTLIFhYrGd6nXOiLpTwajWihZPkoSwkpAG4 KMb1wBR8gQ6k3vZny6HJPL5H7pdfU273SLQgvwVDMmo+ItskAioYOvKEawlCaxyoFpm/ roHw== 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=PgWFbOvfoB6c+12vVAFa3Lz4ErLVnkwWwwAyfBQPr2w=; b=lcELYmzVffKjTCBIIIzm/YBONqTvxDKdPF0l5A/Vm+3eKX+XadXeiNKK7VfaCVXfea RJBh185tnHHIiBobnBuAGtmplmrdZ2kXfWlDiLlQO24C0MagQoGwrQVCjlyUt0sbjV4w jKvUB53VUSuUDbh+Pa1PoLFtBQucKmwIKBxEc0WVjehHSgQPyyzaGCn7s2yfpHuyM3jz ONyQs1PjxFP++N7dmGGI9rciGqsUGT8/Ko326e6dpYRrlC9zH/qN/r2nrgmWV326WvU7 NEt1vvLqqmfGK9o2mS8XUNJg282EsCLsuMPJr9k7uvk9JZpQ0B9fbniDFq/ifEV+i5lo SWQg== X-Gm-Message-State: APjAAAXRJkIsY1gOIi2e3PRbTu1C0lmcqARGoQA1ZCtdyyz+XWeVqDUG zwavMjEswYPDgFw5Wer5h5g2YMxt9sL/Cv1EbGCpjQ== X-Received: by 2002:ab0:1a5:: with SMTP id 34mr6865486ual.15.1551916836116; Wed, 06 Mar 2019 16:00:36 -0800 (PST) Date: Wed, 6 Mar 2019 15:59:12 -0800 In-Reply-To: <20190306235913.6631-1-matthewgarrett@google.com> Message-Id: <20190306235913.6631-27-matthewgarrett@google.com> Mime-Version: 1.0 References: <20190306235913.6631-1-matthewgarrett@google.com> X-Mailer: git-send-email 2.21.0.352.gf09ad66450-goog Subject: [PATCH 26/27] 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 Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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); -- 2.21.0.352.gf09ad66450-goog