From a068acf2ee77693e0bf39d6e07139ba704f461c3 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Fri, 4 Sep 2015 15:44:57 -0700 Subject: 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 Acked-by: Serge Hallyn Acked-by: Jan Kara Acked-by: Paul Moore Cc: J. R. Okajima Signed-off-by: Kees Cook Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/ext4/super.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'fs/ext4') 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 } -- cgit v1.2.3