linux-kernel - [RFC PATCH v2 11/14] vfs: Wire up security hooks for lsm-based device guard in userns

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Message-Id: <20231018105033.13669-12-michael.weiss@aisec.fraunhofer.de>
Date:   Wed, 18 Oct 2023 12:50:30 +0200
From:   Michael Weiß <michael.weiss@...ec.fraunhofer.de>
To:     Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Christian Brauner <brauner@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Paul Moore <paul@...l-moore.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Quentin Monnet <quentin@...valent.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Amir Goldstein <amir73il@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        gyroidos@...ec.fraunhofer.de,
        Michael Weiß <michael.weiss@...ec.fraunhofer.de>
Subject: [RFC PATCH v2 11/14] vfs: Wire up security hooks for lsm-based device guard in userns

Wire up security_inode_mknod_capns() in fs/namei.c. If implemented
and access is granted by an lsm, check ns_capable() instead of the
global CAP_MKNOD.

Wire up security_sb_alloc_userns() in fs/super.c. If implemented
and access is granted by an lsm, the created super block will allow
access to device nodes also if it was created in a non-inital userns.

Signed-off-by: Michael Weiß <michael.weiss@...ec.fraunhofer.de>
---
 fs/namei.c | 16 +++++++++++++++-
 fs/super.c |  6 +++++-
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index f601fcbdc4d2..1f68d160e2c0 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3949,6 +3949,20 @@ inline struct dentry *user_path_create(int dfd, const char __user *pathname,
 }
 EXPORT_SYMBOL(user_path_create);
 
+static bool mknod_capable(struct inode *dir, struct dentry *dentry,
+			  umode_t mode, dev_t dev)
+{
+	/*
+	 * In case of a security hook implementation check mknod in user
+	 * namespace. Otherwise just check global capability.
+	 */
+	int error = security_inode_mknod_nscap(dir, dentry, mode, dev);
+	if (!error)
+		return ns_capable(current_user_ns(), CAP_MKNOD);
+	else
+		return capable(CAP_MKNOD);
+}
+
 /**
  * vfs_mknod - create device node or file
  * @idmap:	idmap of the mount the inode was found from
@@ -3975,7 +3989,7 @@ int vfs_mknod(struct mnt_idmap *idmap, struct inode *dir,
 		return error;
 
 	if ((S_ISCHR(mode) || S_ISBLK(mode)) && !is_whiteout &&
-	    !capable(CAP_MKNOD))
+	    !mknod_capable(dir, dentry, mode, dev))
 		return -EPERM;
 
 	if (!dir->i_op->mknod)
diff --git a/fs/super.c b/fs/super.c
index 2d762ce67f6e..bb01db6d9986 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -362,7 +362,11 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	}
 	s->s_bdi = &noop_backing_dev_info;
 	s->s_flags = flags;
-	if (s->s_user_ns != &init_user_ns)
+	/*
+	 * We still have to think about this here. Several concerns exist
+	 * about the security model, especially about malicious fuse.
+	 */
+	if (s->s_user_ns != &init_user_ns && security_sb_alloc_userns(s))
 		s->s_iflags |= SB_I_NODEV;
 	INIT_HLIST_NODE(&s->s_instances);
 	INIT_HLIST_BL_HEAD(&s->s_roots);
-- 
2.30.2