[PATCH 3.2 065/107] fs: create and use seq_show_option for escaping

[Ben Hutchings <ben@decadent.org.uk>]

3.2.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Kees Cook <keescook@chromium.org>

commit a068acf2ee77693e0bf39d6e07139ba704f461c3 upstream.

Many file systems that implement the show_options hook fail to correctly
escape their output which could lead to unescaped characters (e.g.  new
lines) leaking into /proc/mounts and /proc/[pid]/mountinfo files.  This
could lead to confusion, spoofed entries (resulting in things like
systemd issuing false d-bus "mount" notifications), and who knows what
else.  This looks like it would only be the root user stepping on
themselves, but it's possible weird things could happen in containers or
in other situations with delegated mount privileges.

Here's an example using overlay with setuid fusermount trusting the
contents of /proc/mounts (via the /etc/mtab symlink).  Imagine the use
of "sudo" is something more sneaky:

  $ BASE="ovl"
  $ MNT="$BASE/mnt"
  $ LOW="$BASE/lower"
  $ UP="$BASE/upper"
  $ WORK="$BASE/work/ 0 0
  none /proc fuse.pwn user_id=1000"
  $ mkdir -p "$LOW" "$UP" "$WORK"
  $ sudo mount -t overlay -o "lowerdir=$LOW,upperdir=$UP,workdir=$WORK" none /mnt
  $ cat /proc/mounts
  none /root/ovl/mnt overlay rw,relatime,lowerdir=ovl/lower,upperdir=ovl/upper,workdir=ovl/work/ 0 0
  none /proc fuse.pwn user_id=1000 0 0
  $ fusermount -u /proc
  $ cat /proc/mounts
  cat: /proc/mounts: No such file or directory

This fixes the problem by adding new seq_show_option and
seq_show_option_n helpers, and updating the vulnerable show_option
handlers to use them as needed.  Some, like SELinux, need to be open
coded due to unusual existing escape mechanisms.

[akpm@linux-foundation.org: add lost chunk, per Kees]
[keescook@chromium.org: seq_show_option should be using const parameters]
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Acked-by: Jan Kara <jack@suse.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Cc: J. R. Okajima <hooanon05g@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2:
 - Drop changes to overlayfs, reiserfs
 - Drop vers option from cifs
 - ceph changes are all in one file
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -361,8 +361,10 @@ static int ceph_show_options(struct seq_
 	if (opt->flags & CEPH_OPT_NOCRC)
 		seq_puts(m, ",nocrc");
 
-	if (opt->name)
-		seq_printf(m, ",name=%s", opt->name);
+	if (opt->name) {
+		seq_puts(m, ",name=");
+		seq_escape(m, opt->name, ", \t\n\\");
+	}
 	if (opt->key)
 		seq_puts(m, ",secret=<hidden>");
 
@@ -405,7 +407,7 @@ static int ceph_show_options(struct seq_
 	if (fsopt->max_readdir_bytes != CEPH_MAX_READDIR_BYTES_DEFAULT)
 		seq_printf(m, ",readdir_max_bytes=%d", fsopt->max_readdir_bytes);
 	if (strcmp(fsopt->snapdir_name, CEPH_SNAPDIRNAME_DEFAULT))
-		seq_printf(m, ",snapdirname=%s", fsopt->snapdir_name);
+		seq_show_option(m, "snapdirname", fsopt->snapdir_name);
 	return 0;
 }
 
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -381,10 +381,10 @@ cifs_show_options(struct seq_file *s, st
 	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_MULTIUSER)
 		seq_printf(s, ",multiuser");
 	else if (tcon->ses->user_name)
-		seq_printf(s, ",username=%s", tcon->ses->user_name);
+		seq_show_option(s, "username", tcon->ses->user_name);
 
 	if (tcon->ses->domainName)
-		seq_printf(s, ",domain=%s", tcon->ses->domainName);
+		seq_show_option(s, "domain", tcon->ses->domainName);
 
 	if (srcaddr->sa_family != AF_UNSPEC) {
 		struct sockaddr_in *saddr4;
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1018,10 +1018,10 @@ static inline void ext4_show_quota_optio
 	}
 
 	if (sbi->s_qf_names[USRQUOTA])
-		seq_printf(seq, ",usrjquota=%s", sbi->s_qf_names[USRQUOTA]);
+		seq_show_option(seq, "usrjquota", sbi->s_qf_names[USRQUOTA]);
 
 	if (sbi->s_qf_names[GRPQUOTA])
-		seq_printf(seq, ",grpjquota=%s", sbi->s_qf_names[GRPQUOTA]);
+		seq_show_option(seq, "grpjquota", sbi->s_qf_names[GRPQUOTA]);
 
 	if (test_opt(sb, USRQUOTA))
 		seq_puts(seq, ",usrquota");
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -1298,11 +1298,11 @@ static int gfs2_show_options(struct seq_
 	if (is_ancestor(mnt->mnt_root, sdp->sd_master_dir))
 		seq_printf(s, ",meta");
 	if (args->ar_lockproto[0])
-		seq_printf(s, ",lockproto=%s", args->ar_lockproto);
+		seq_show_option(s, "lockproto", args->ar_lockproto);
 	if (args->ar_locktable[0])
-		seq_printf(s, ",locktable=%s", args->ar_locktable);
+		seq_show_option(s, "locktable", args->ar_locktable);
 	if (args->ar_hostdata[0])
-		seq_printf(s, ",hostdata=%s", args->ar_hostdata);
+		seq_show_option(s, "hostdata", args->ar_hostdata);
 	if (args->ar_spectator)
 		seq_printf(s, ",spectator");
 	if (args->ar_localflocks)
--- a/fs/hfs/super.c
+++ b/fs/hfs/super.c
@@ -138,9 +138,9 @@ static int hfs_show_options(struct seq_f
 	struct hfs_sb_info *sbi = HFS_SB(mnt->mnt_sb);
 
 	if (sbi->s_creator != cpu_to_be32(0x3f3f3f3f))
-		seq_printf(seq, ",creator=%.4s", (char *)&sbi->s_creator);
+		seq_show_option_n(seq, "creator", (char *)&sbi->s_creator, 4);
 	if (sbi->s_type != cpu_to_be32(0x3f3f3f3f))
-		seq_printf(seq, ",type=%.4s", (char *)&sbi->s_type);
+		seq_show_option_n(seq, "type", (char *)&sbi->s_type, 4);
 	seq_printf(seq, ",uid=%u,gid=%u", sbi->s_uid, sbi->s_gid);
 	if (sbi->s_file_umask != 0133)
 		seq_printf(seq, ",file_umask=%o", sbi->s_file_umask);
--- a/fs/hfsplus/options.c
+++ b/fs/hfsplus/options.c
@@ -211,9 +211,9 @@ int hfsplus_show_options(struct seq_file
 	struct hfsplus_sb_info *sbi = HFSPLUS_SB(mnt->mnt_sb);
 
 	if (sbi->creator != HFSPLUS_DEF_CR_TYPE)
-		seq_printf(seq, ",creator=%.4s", (char *)&sbi->creator);
+		seq_show_option_n(seq, "creator", (char *)&sbi->creator, 4);
 	if (sbi->type != HFSPLUS_DEF_CR_TYPE)
-		seq_printf(seq, ",type=%.4s", (char *)&sbi->type);
+		seq_show_option_n(seq, "type", (char *)&sbi->type, 4);
 	seq_printf(seq, ",umask=%o,uid=%u,gid=%u", sbi->umask,
 		sbi->uid, sbi->gid);
 	if (sbi->part >= 0)
--- a/fs/hostfs/hostfs_kern.c
+++ b/fs/hostfs/hostfs_kern.c
@@ -265,7 +265,7 @@ static int hostfs_show_options(struct se
 	size_t offset = strlen(root_ino) + 1;
 
 	if (strlen(root_path) > offset)
-		seq_printf(seq, ",%s", root_path + offset);
+		seq_show_option(seq, root_path + offset, NULL);
 
 	return 0;
 }
--- a/fs/ocfs2/super.c
+++ b/fs/ocfs2/super.c
@@ -1583,8 +1583,8 @@ static int ocfs2_show_options(struct seq
 		seq_printf(s, ",localflocks,");
 
 	if (osb->osb_cluster_stack[0])
-		seq_printf(s, ",cluster_stack=%.*s", OCFS2_STACK_LABEL_LEN,
-			   osb->osb_cluster_stack);
+		seq_show_option_n(s, "cluster_stack", osb->osb_cluster_stack,
+				  OCFS2_STACK_LABEL_LEN);
 	if (opts & OCFS2_MOUNT_USRQUOTA)
 		seq_printf(s, ",usrquota");
 	if (opts & OCFS2_MOUNT_GRPQUOTA)
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -533,9 +533,9 @@ xfs_showargs(
 		seq_printf(m, "," MNTOPT_LOGBSIZE "=%dk", mp->m_logbsize >> 10);
 
 	if (mp->m_logname)
-		seq_printf(m, "," MNTOPT_LOGDEV "=%s", mp->m_logname);
+		seq_show_option(m, MNTOPT_LOGDEV, mp->m_logname);
 	if (mp->m_rtname)
-		seq_printf(m, "," MNTOPT_RTDEV "=%s", mp->m_rtname);
+		seq_show_option(m, MNTOPT_RTDEV, mp->m_rtname);
 
 	if (mp->m_dalign > 0)
 		seq_printf(m, "," MNTOPT_SUNIT "=%d",
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -122,6 +122,41 @@ void *__seq_open_private(struct file *,
 int seq_open_private(struct file *, const struct seq_operations *, int);
 int seq_release_private(struct inode *, struct file *);
 
+/**
+ * seq_show_options - display mount options with appropriate escapes.
+ * @m: the seq_file handle
+ * @name: the mount option name
+ * @value: the mount option name's value, can be NULL
+ */
+static inline void seq_show_option(struct seq_file *m, const char *name,
+				   const char *value)
+{
+	seq_putc(m, ',');
+	seq_escape(m, name, ",= \t\n\\");
+	if (value) {
+		seq_putc(m, '=');
+		seq_escape(m, value, ", \t\n\\");
+	}
+}
+
+/**
+ * seq_show_option_n - display mount options with appropriate escapes
+ *		       where @value must be a specific length.
+ * @m: the seq_file handle
+ * @name: the mount option name
+ * @value: the mount option name's value, cannot be NULL
+ * @length: the length of @value to display
+ *
+ * This is a macro since this uses "length" to define the size of the
+ * stack buffer.
+ */
+#define seq_show_option_n(m, name, value, length) {	\
+	char val_buf[length + 1];			\
+	strncpy(val_buf, value, length);		\
+	val_buf[length] = '\0';				\
+	seq_show_option(m, name, val_buf);		\
+}
+
 #define SEQ_START_TOKEN ((void *)1)
 
 /*
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1053,15 +1053,16 @@ static int cgroup_show_options(struct se
 
 	mutex_lock(&cgroup_mutex);
 	for_each_subsys(root, ss)
-		seq_printf(seq, ",%s", ss->name);
+		seq_show_option(seq, ss->name, NULL);
 	if (test_bit(ROOT_NOPREFIX, &root->flags))
 		seq_puts(seq, ",noprefix");
 	if (strlen(root->release_agent_path))
-		seq_printf(seq, ",release_agent=%s", root->release_agent_path);
+		seq_show_option(seq, "release_agent",
+				root->release_agent_path);
 	if (clone_children(&root->top_cgroup))
 		seq_puts(seq, ",clone_children");
 	if (strlen(root->name))
-		seq_printf(seq, ",name=%s", root->name);
+		seq_show_option(seq, "name", root->name);
 	mutex_unlock(&cgroup_mutex);
 	return 0;
 }
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1011,7 +1011,7 @@ static void selinux_write_opts(struct se
 		seq_puts(m, prefix);
 		if (has_comma)
 			seq_putc(m, '\"');
-		seq_puts(m, opts->mnt_opts[i]);
+		seq_escape(m, opts->mnt_opts[i], "\"\n\\");
 		if (has_comma)
 			seq_putc(m, '\"');
 	}