fs: create and use seq_show_option for escaping

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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

15 files changed, 71 insertions, 30 deletions

diff --git a/fs/ceph/super.c b/fs/ceph/super.c
index d1c833c..7b6bfcb 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -479,7 +479,7 @@ static int ceph_show_options(struct seq_file *m, struct dentry *root)
 	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;
 }
diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 0a9fb6b..6a1119e 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -394,17 +394,17 @@ cifs_show_options(struct seq_file *s, struct dentry *root)
 	struct sockaddr *srcaddr;
 	srcaddr = (struct sockaddr *)&tcon->ses->server->srcaddr;
 
-	seq_printf(s, ",vers=%s", tcon->ses->server->vals->version_string);
+	seq_show_option(s, "vers", tcon->ses->server->vals->version_string);
 	cifs_show_security(s, tcon->ses);
 	cifs_show_cache_flavor(s, cifs_sb);
 
 	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_MULTIUSER)
 		seq_puts(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;
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ee38782..a63c7b0 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1776,10 +1776,10 @@ static inline void ext4_show_quota_options(struct seq_file *seq,
 	}
 
 	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]);
 #endif
 }
 
diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
index 2982445..894fb01 100644
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -1334,11 +1334,11 @@ static int gfs2_show_options(struct seq_file *s, struct dentry *root)
 	if (is_ancestor(root, sdp->sd_master_dir))
 		seq_puts(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_puts(s, ",spectator");
 	if (args->ar_localflocks)
diff --git a/fs/hfs/super.c b/fs/hfs/super.c
index 55c03b9..4574fdd 100644
--- a/fs/hfs/super.c
+++ b/fs/hfs/super.c
@@ -136,9 +136,9 @@ static int hfs_show_options(struct seq_file *seq, struct dentry *root)
 	struct hfs_sb_info *sbi = HFS_SB(root->d_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",
 			from_kuid_munged(&init_user_ns, sbi->s_uid),
 			from_kgid_munged(&init_user_ns, sbi->s_gid));
diff --git a/fs/hfsplus/options.c b/fs/hfsplus/options.c
index c90b72e..bb806e5 100644
--- a/fs/hfsplus/options.c
+++ b/fs/hfsplus/options.c
@@ -218,9 +218,9 @@ int hfsplus_show_options(struct seq_file *seq, struct dentry *root)
 	struct hfsplus_sb_info *sbi = HFSPLUS_SB(root->d_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,
 			from_kuid_munged(&init_user_ns, sbi->uid),
 			from_kgid_munged(&init_user_ns, sbi->gid));
diff --git a/fs/hostfs/hostfs_kern.c b/fs/hostfs/hostfs_kern.c
index 059597b..2ac99db 100644
--- a/fs/hostfs/hostfs_kern.c
+++ b/fs/hostfs/hostfs_kern.c
@@ -260,7 +260,7 @@ static int hostfs_show_options(struct seq_file *seq, struct dentry *root)
 	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);
 
 	if (append)
 		seq_puts(seq, ",append");
diff --git a/fs/ocfs2/super.c b/fs/ocfs2/super.c
index 3a9a1af..2de4c8a 100644
--- a/fs/ocfs2/super.c
+++ b/fs/ocfs2/super.c
@@ -1563,8 +1563,8 @@ static int ocfs2_show_options(struct seq_file *s, struct dentry *root)
 		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)
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 7466ff3..79073d6 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -588,10 +588,10 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 	struct super_block *sb = dentry->d_sb;
 	struct ovl_fs *ufs = sb->s_fs_info;
 
-	seq_printf(m, ",lowerdir=%s", ufs->config.lowerdir);
+	seq_show_option(m, "lowerdir", ufs->config.lowerdir);
 	if (ufs->config.upperdir) {
-		seq_printf(m, ",upperdir=%s", ufs->config.upperdir);
-		seq_printf(m, ",workdir=%s", ufs->config.workdir);
+		seq_show_option(m, "upperdir", ufs->config.upperdir);
+		seq_show_option(m, "workdir", ufs->config.workdir);
 	}
 	return 0;
 }
diff --git a/fs/reiserfs/super.c b/fs/reiserfs/super.c
index 0e4cf728..4a62fe8 100644
--- a/fs/reiserfs/super.c
+++ b/fs/reiserfs/super.c
@@ -714,18 +714,20 @@ static int reiserfs_show_options(struct seq_file *seq, struct dentry *root)
 		seq_puts(seq, ",acl");
 
 	if (REISERFS_SB(s)->s_jdev)
-		seq_printf(seq, ",jdev=%s", REISERFS_SB(s)->s_jdev);
+		seq_show_option(seq, "jdev", REISERFS_SB(s)->s_jdev);
 
 	if (journal->j_max_commit_age != journal->j_default_max_commit_age)
 		seq_printf(seq, ",commit=%d", journal->j_max_commit_age);
 
 #ifdef CONFIG_QUOTA
 	if (REISERFS_SB(s)->s_qf_names[USRQUOTA])
-		seq_printf(seq, ",usrjquota=%s", REISERFS_SB(s)->s_qf_names[USRQUOTA]);
+		seq_show_option(seq, "usrjquota",
+				REISERFS_SB(s)->s_qf_names[USRQUOTA]);
 	else if (opts & (1 << REISERFS_USRQUOTA))
 		seq_puts(seq, ",usrquota");
 	if (REISERFS_SB(s)->s_qf_names[GRPQUOTA])
-		seq_printf(seq, ",grpjquota=%s", REISERFS_SB(s)->s_qf_names[GRPQUOTA]);
+		seq_show_option(seq, "grpjquota",
+				REISERFS_SB(s)->s_qf_names[GRPQUOTA]);
 	else if (opts & (1 << REISERFS_GRPQUOTA))
 		seq_puts(seq, ",grpquota");
 	if (REISERFS_SB(s)->s_jquota_fmt) {
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 1fb16562..bbd9b1f 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -511,9 +511,9 @@ 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",
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index 912a7c4..d4c7271 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -149,6 +149,41 @@ static inline struct user_namespace *seq_user_ns(struct seq_file *seq)
 #endif
 }
 
+/**
+ * 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)
 /*
  * Helpers for iteration over list_head-s in seq_files
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f3f5cd5..a8538e4 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1342,7 +1342,7 @@ static int cgroup_show_options(struct seq_file *seq,
 	if (root != &cgrp_dfl_root)
 		for_each_subsys(ss, ssid)
 			if (root->subsys_mask & (1 << ssid))
-				seq_printf(seq, ",%s", ss->legacy_name);
+				seq_show_option(seq, ss->name, NULL);
 	if (root->flags & CGRP_ROOT_NOPREFIX)
 		seq_puts(seq, ",noprefix");
 	if (root->flags & CGRP_ROOT_XATTR)
@@ -1350,13 +1350,14 @@ static int cgroup_show_options(struct seq_file *seq,
 
 	spin_lock(&release_agent_path_lock);
 	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);
 	spin_unlock(&release_agent_path_lock);
 
 	if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->cgrp.flags))
 		seq_puts(seq, ",clone_children");
 	if (strlen(root->name))
-		seq_printf(seq, ",name=%s", root->name);
+		seq_show_option(seq, "name", root->name);
 	return 0;
 }
 
diff --git a/net/ceph/ceph_common.c b/net/ceph/ceph_common.c
index f30329f..69a4d30 100644
--- a/net/ceph/ceph_common.c
+++ b/net/ceph/ceph_common.c
@@ -517,8 +517,11 @@ int ceph_print_client_options(struct seq_file *m, struct ceph_client *client)
 	struct ceph_options *opt = client->options;
 	size_t pos = m->count;
 
-	if (opt->name)
-		seq_printf(m, "name=%s,", opt->name);
+	if (opt->name) {
+		seq_puts(m, "name=");
+		seq_escape(m, opt->name, ", \t\n\\");
+		seq_putc(m, ',');
+	}
 	if (opt->key)
 		seq_puts(m, "secret=<hidden>,");
 
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 564079c..cdf4c58 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1100,7 +1100,7 @@ static void selinux_write_opts(struct seq_file *m,
 		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, '\"');
 	}