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 d1c833c321b9..7b6bfcbf801c 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 0a9fb6b53126..6a1119e87fbb 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 ee3878262a49..a63c7b0a10cf 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 2982445947e1..894fb01a91da 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 55c03b9e9070..4574fdd3d421 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 c90b72ee676d..bb806e58c977 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 059597b23f67..2ac99db3750e 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 3a9a1af39ad7..2de4c8a9340c 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 7466ff339c66..79073d68b475 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 0e4cf728126f..4a62fe8cc3bf 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 1fb16562c159..bbd9b1f10ffb 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 912a7c482649..d4c7271382cb 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 f3f5cd5e2c0d..a8538e443784 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 f30329f72641..69a4d30a9ccf 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 564079c5c49d..cdf4c589a391 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, '\"');
 	}