bpfilter blocks root unmount during shutdown

Andrei Borzenkov
2018-09-23T07:38:02Z

Dracut /shutdown script first tries to kill all processes still running
off old root. Unfortunately this fails for special user process that
runs bpfilter because it does not include reference to /oldroot in
places where dracut looks for in kilall_proc_mountpoint()

10:~ # ps -ef | fgrep '[none]'
root 984 2 0 09:46 ? 00:00:00 [none]

/proc/984:
total 0
dr-xr-xr-x 2 root 0 0 Sep 23 10:11 attr
-r-------- 1 root 0 0 Sep 23 10:11 auxv
-r--r--r-- 1 root 0 0 Sep 23 10:11 cgroup
--w------- 1 root 0 0 Sep 23 10:11 clear_refs
-r--r--r-- 1 root 0 0 Sep 23 10:10 cmdline
-rw-r--r-- 1 root 0 0 Sep 23 10:11 comm
-rw-r--r-- 1 root 0 0 Sep 23 10:11 coredump_filter
-r--r--r-- 1 root 0 0 Sep 23 10:11 cpuset
lrwxrwxrwx 1 root 0 0 Sep 23 10:11 cwd -> /
-r-------- 1 root 0 0 Sep 23 10:11 environ
lrwxrwxrwx 1 root 0 0 Sep 23 10:11 exe -> / (deleted)
-rw-r--r-- 1 root 0 0 Sep 23 10:11 fail-nth
dr-x------ 2 root 0 0 Sep 23 10:11 fd
dr-x------ 2 root 0 0 Sep 23 10:11 fdinfo
-rw-r--r-- 1 root 0 0 Sep 23 10:11 gid_map
-r-------- 1 root 0 0 Sep 23 10:11 io
-r--r--r-- 1 root 0 0 Sep 23 10:11 latency
-r--r--r-- 1 root 0 0 Sep 23 10:11 limits
-rw-r--r-- 1 root 0 0 Sep 23 10:11 loginuid
-rw-r--r-- 1 root 0 0 Sep 23 10:11 make-it-fail
dr-x------ 2 root 0 0 Sep 23 10:11 map_files
-r--r--r-- 1 root 0 0 Sep 23 10:10 maps
-rw------- 1 root 0 0 Sep 23 10:11 mem
-r--r--r-- 1 root 0 0 Sep 23 10:11 mountinfo
-r--r--r-- 1 root 0 0 Sep 23 10:11 mounts
-r-------- 1 root 0 0 Sep 23 10:11 mountstats
dr-xr-xr-x 6 root 0 0 Sep 23 10:11 net
dr-x--x--x 2 root 0 0 Sep 23 10:11 ns
-r--r--r-- 1 root 0 0 Sep 23 10:11 numa_maps
-rw-r--r-- 1 root 0 0 Sep 23 10:11 oom_adj
-r--r--r-- 1 root 0 0 Sep 23 10:11 oom_score
-rw-r--r-- 1 root 0 0 Sep 23 10:11 oom_score_adj
-r-------- 1 root 0 0 Sep 23 10:11 pagemap
-r-------- 1 root 0 0 Sep 23 10:11 patch_state
-r-------- 1 root 0 0 Sep 23 10:11 personality
-rw-r--r-- 1 root 0 0 Sep 23 10:11 projid_map
lrwxrwxrwx 1 root 0 0 Sep 23 10:11 root -> /
-rw-r--r-- 1 root 0 0 Sep 23 10:11 sched
-r--r--r-- 1 root 0 0 Sep 23 10:11 schedstat
-r--r--r-- 1 root 0 0 Sep 23 10:11 sessionid
-rw-r--r-- 1 root 0 0 Sep 23 10:11 setgroups
-r--r--r-- 1 root 0 0 Sep 23 10:11 smaps
-r--r--r-- 1 root 0 0 Sep 23 10:11 smaps_rollup
-r-------- 1 root 0 0 Sep 23 10:11 stack
-r--r--r-- 1 root 0 0 Sep 23 10:10 stat
-r--r--r-- 1 root 0 0 Sep 23 10:11 statm
-r--r--r-- 1 root 0 0 Sep 23 10:10 status
-r-------- 1 root 0 0 Sep 23 10:11 syscall
dr-xr-xr-x 3 root 0 0 Sep 23 10:11 task
-r--r--r-- 1 root 0 0 Sep 23 10:11 timers
-rw-rw-rw- 1 root 0 0 Sep 23 10:11 timerslack_ns
-rw-r--r-- 1 root 0 0 Sep 23 10:11 uid_map
-r--r--r-- 1 root 0 0 Sep 23 10:11 wchan

/proc/984/fd:
total 0
lr-x------ 1 root 0 64 Sep 23 10:11 0 -> pipe:[19409]
l-wx------ 1 root 0 64 Sep 23 10:11 1 -> pipe:[19410]
lrwx------ 1 root 0 64 Sep 23 10:11 2 -> /oldsys/dev/console

But it does contain reference to /oldroot in its mapped libraries list
(/proc/984/maps):

563b63002000-563b63003000 r--p 00000000 00:05 19404
/ (deleted)
563b63003000-563b63004000 r-xp 00001000 00:05 19404
/ (deleted)
563b63004000-563b63005000 r--p 00002000 00:05 19404
/ (deleted)
563b63005000-563b63006000 r--p 00002000 00:05 19404
/ (deleted)
563b63006000-563b63007000 rw-p 00003000 00:05 19404
/ (deleted)
563b63fb4000-563b63fd5000 rw-p 00000000 00:00 0
[heap]
7fa3a46cc000-7fa3a4882000 r-xp 00000000 00:2a 7728
/oldroot/lib64/libc-2.27.so
7fa3a4882000-7fa3a4a82000 ---p 001b6000 00:2a 7728
/oldroot/lib64/libc-2.27.so
7fa3a4a82000-7fa3a4a86000 r--p 001b6000 00:2a 7728
/oldroot/lib64/libc-2.27.so
7fa3a4a86000-7fa3a4a88000 rw-p 001ba000 00:2a 7728
/oldroot/lib64/libc-2.27.so
7fa3a4a88000-7fa3a4a8c000 rw-p 00000000 00:00 0
7fa3a4a8c000-7fa3a4ab1000 r-xp 00000000 00:2a 7720
/oldroot/lib64/ld-2.27.so
7fa3a4ca7000-7fa3a4ca9000 rw-p 00000000 00:00 0
7fa3a4cb1000-7fa3a4cb2000 r--p 00025000 00:2a 7720
/oldroot/lib64/ld-2.27.so
7fa3a4cb2000-7fa3a4cb3000 rw-p 00026000 00:2a 7720
/oldroot/lib64/ld-2.27.so
7fa3a4cb3000-7fa3a4cb4000 rw-p 00000000 00:00 0
7ffea03b4000-7ffea03d5000 rw-p 00000000 00:00 0
[stack]
7ffea03df000-7ffea03e2000 r--p 00000000 00:00 0
[vvar]
7ffea03e2000-7ffea03e4000 r-xp 00000000 00:00 0
[vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0
[vsyscall]

So the quick fix would be to extend check for root references to also
look into /proc/$PID/maps. Something like (verified):

--- dracut-lib.sh.orig 2018-09-18 13:24:49.000000000 +0300
+++ dracut-lib.sh 2018-09-23 10:31:13.300054544 +0300
@@ -118,7 +118,7 @@ killall_proc_mountpoint() {
esac
[ -e "/proc/$_pid/exe" ] || continue
[ -e "/proc/$_pid/root" ] || continue
- strstr "$(ls -l -- "/proc/$_pid" "/proc/$_pid/fd" 2>/dev/null)"
"$1" && kill -9 "$_pid"
+ strstr "$(ls -l -- "/proc/$_pid" "/proc/$_pid/fd" 2>/dev/null;
cat "/proc/$_pid/maps" 2> /dev/null)" "$1" && kill -9 "$_pid"
done
}

Note that there are also other places that use similar check (most
obvious being /shutdown script itself) which likely need uniform fix. If
there are no objection, I would introduce helper function to do check
and use it everywhere instead of open coding.