Blog·Tanky WooABOUTRSS

Mount option hidepid with ssh-agent

20 Jun 2017

Problem Description

In my dotfiles, when I start a new bash login shell, I will check if a ssh-agent is already started, if not, start a new ssh-agent:

# Set up ssh-agent

function start_agent {
    echo "Initializing new SSH agent..."
    /usr/bin/ssh-agent | sed 's/^echo/#echo/' > "${SSH_ENV}"
    echo succeeded
    chmod 600 "${SSH_ENV}"
    . "${SSH_ENV}" > /dev/null

# Source SSH settings, if applicable
if [ -f "${SSH_ENV}" ]; then
    . "${SSH_ENV}" > /dev/null
    #ps ${SSH_AGENT_PID} doesn't work under cywgin
    ps -ef | grep ${SSH_AGENT_PID} | grep ssh-agent$ > /dev/null || {

When starting a new ssh-agent, I saved the info (environ variable) to a file, and next time I check the exists of the file, then check if ssh-agent process exists.

Recently, I login a jump server with one people one login user, and found a process owned by other people with command line:

redis-cli -h x.x.x.x -a <password string>

It's obvious that as a common permission user, I can also see other people's process with command line, if someone write password in the command line, not in interactive mode, it is insecure.

Resolve the process list isolation

The solution is mount with option hidepid.

Such as ps, top etc. the process info is from /proc, in the head of man 5 proc document describe mount option hidepid, the default value is 0, and not specified when mount proc directory, all user can see other's processes command line.

With value 1, user can only see his own processes, but can list other's process id under /proc directory.

With value 2, user can only see his own processes, and if he list directory under /proc, he can only list his own pid directory, other pid directory is hidden.

So we can choose 1 or 2 to implement process list isolation:

# runtime
mount -o remount,rw,hidepid=2 /proc

# in /etc/fstab
# <fs>      <mountpoint>   <type>     <opts>               <dump/pass>
proc        /proc          proc       defaults,hidepid=2   0 0

Ok, do this can resolve the process list isolation, but it introduce another problem:

I can't see the ssh-agent process, this cause every time I start a new login shell, I will ask to start a new ssh-agent process with my dotfiles.

From user root:

root $ ps aux | grep ssh-agent
tankywoo 16282  0.0  0.0  19584   344 ?        Ss   18:03   0:00 /usr/bin/ssh-agent

root $ ls -al /proc/16282
ls -al /proc/16282
total 0
dr-xr-xr-x   9 tankywoo tankywoo 0 Jun 20 18:04 .
dr-xr-xr-x 214 root     root     0 Jun 14 02:52 ..
dr-xr-xr-x   2 tankywoo tankywoo 0 Jun 20 18:04 attr
-rw-r--r--   1 root     root     0 Jun 20 18:04 autogroup
-r--r--r--   1 root     root     0 Jun 20 18:04 cmdline
-r--r--r--   1 root     root     0 Jun 20 18:04 stat
-r--r--r--   1 root     root     0 Jun 20 18:04 statm
-r--r--r--   1 root     root     0 Jun 20 18:04 status

From the process's owner tankywoo:

tankywoo $ ls -al /proc/16282
ls: cannot open directory '/proc/16282': Operation not permitted

Why are the files under /proc/[pid] owned by root?

As the result list above, files under /proc/16282 owns to root, not tankywoo.

We can ps aux and find other processes owns by yourself, and the files under /proc/[pid] should owns to yourself.

The I found in man 5 proc:


There is a numerical subdirectory for each running process; the subdirectory is named by the process ID.

Each /proc/[pid] subdirectory contains the pseudo-files and directories described below. These files are normally owned by the effective user and effective group ID of the process. However is made root:root if the process's "dumpable" attribute is set to a value other than 1. This attribute may change for the following reasons:

  • The attribute was explicitly set via the prctl(2) PR_SET_DUMPABLE operation.

  • The attribute was reset to the value in the file /proc/sys/fs/suid_dumpable (described below), for the reasons described in prctl(2).

Resetting the "dumpable" attribute to 1 reverts the ownership of the /proc/[pid]/* files to the process's real UID and real GID.

As above said, if the process's dumpable attribute is set with value not equal 1, the files will be owned by root.

More with man 2 prctl:

PRSETDUMPABLE (since Linux 2.3.20)

Set the state of the "dumpable" flag, which determines whether core dumps are produced for the calling process upon delivery of a signal whose default behavior is to produce a core dump.

In kernels up to and including 2.6.12, arg2 must be either 0 (SUID_DUMP_DISABLE, process is not dumpable) or 1 (SUID_DUMP_USER, process is dumpable). Between kernels 2.6.13 and 2.6.17, the value 2 was also permitted, which caused any binary which normally would not be dumped to be dumped readable by root only; for security reasons, this feature has been removed. (See also the description of /proc/sys/fs/suid_dumpable in proc(5).)

Normally, this flag is set to 1. However, it is reset to the current value contained in the file /proc/sys/fs/suid_dumpable (which by default has the value 0), in the following circumstances:

  • The process's effective user or group ID is changed.

  • The process's filesystem user or group ID is changed (see credentials(7)).

  • The process executes (execve(2)) a set-user-ID or set-group-ID program, or a program that has capabilities (see capabilities(7)).

Processes that are not dumpable can not be attached via ptrace(2) PTRACE_ATTACH; see ptrace(2) for further details.

If a process is not dumpable, the ownership of files in the process's /proc/[pid] directory is affected as described in proc(5).

Finally, I read the source code of ssh-agent.c, my openssh version is openssh-7.5p1.

Line 1421 of ssh-agent.c, for security, ssh-agent set ulimit's core dump to 0 to deny it.

  /* deny core dumps, since memory contains unencrypted private keys */
  rlim.rlim_cur = rlim.rlim_max = 0;
  if (setrlimit(RLIMIT_CORE, &rlim) < 0) {
      error("setrlimit RLIMIT_CORE: %s", strerror(errno));

and line 1234, ssh-agent set PR_SET_DUMPABLE:

// ssh-agent.c L1234

// platform-tracing.c L33
platform_disable_tracing(int strict)
#if defined(HAVE_PRCTL) && defined(PR_SET_DUMPABLE)
    /* Disable ptrace on Linux without sgid bit */
    if (prctl(PR_SET_DUMPABLE, 0) != 0 && strict)
        fatal("unable to make the process undumpable");

Refer to the man document, prctl(PR_SET_DUMPABLE, 0) cause the files under /proc/[pid] owned by user root.

Why I can't view /proc/[pid] of ssh-agent?

From man 2 ptrace, it list the algorithm of access to /proc/[pid]:

  1. Deny access if the target process "dumpable" attribute has a value other than 1 (SUID_DUMP_USER; see the discussion of PR_SET_DUMPABLE in prctl(2)), and the caller does not have the CAP_SYS_PTRACE capability in the user namespace of the target process.

How to resolve the deny access of ssh-agent?

No solution I can think of at the moment...

In sys/prctl.h defined with #define PR_SET_DUMPABLE 4, unless we change the make of source code...

It is a sad story...