Entries tagged kernel-programming

Related tags: linux-security-module, lsm, lwn, security.

Yet more linux security module craziness ..

Thursday, 29 June 2017

I've recently been looking at linux security modules. My first two experiments helped me learn:

My First module - whitelist_lsm.c

This looked for the presence of an xattr, and if present allowed execution of binaries.

I learned about the Kernel build-system, and how to write a simple LSM.

My second module - hashcheck_lsm.c

This looked for the presence of a "known-good" SHA1 hash xattr, and if it matched the actual hash of the file on-disk allowed execution.

I learned how to hash the contents of a file, from kernel-space.

Both allowed me to learn things, but both were a little pointless. They were not fine-grained enough to allow different things to be done by different users. (i.e. If you allowed "alice" to run "wget" you'd also allow www-data to do the same.)

So, assuming you wanted to do your security job more neatly what would you want? You'd want to allow/deny execution of commands based upon:

  • The user who was invoking them.
  • The path of the binary itself.

So your local users could run "bad" commands, but "www-data" (post-compromise) couldn't.

Obviously you don't want to have to recompile your kernel to change the rules of who can execute what. So you think to yourself "I'll write those rules down in a file". But of course reading a file from kernel-space is tricky. And parsing any list of rules, in a file, from kernel-space would prone to buffer-related problems.

So I had a crazy idea:

  • When a user attempts to execute a program.
  • Call back to user-space to see if that should be permitted.
    • Give the user-space binary the UID of the invoker, and the path to the command they're trying to execute.

Calling userspace? Every time a command is to be executed? Crazy. But it just might work.

One problem I had with this approach is that userspace might not even be available, when you're booting. So I setup a flag to enable this stuff:

# echo 1 >/proc/sys/kernel/can-exec/enabled

Now the kernel will invoke the following on every command:

/sbin/can-exec $UID $PATH

Because the kernel waits for this command to complete - as it reads the exit-code - you cannot execute any child-processes from it as you'd end up in recursive hell, but you can certainly read files, write to syslog, etc. My initial implementionation was as basic as this:

int main( int argc, char *argv[] )
{
...

  // Get the UID + Program
  int uid = atoi( argv[1] );
  char *prg = argv[2];

  // syslog
  openlog ("can-exec", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);
  syslog (LOG_NOTICE, "UID:%d CMD:%s", uid, prg );

  // root can do all.
  if ( uid == 0 )
    return 0;

  // nobody
  if ( uid == 65534 ) {
    if ( ( strcmp( prg , "/bin/sh" ) == 0 ) ||
         ( strcmp( prg , "/usr/bin/id" ) == 0 ) ) {
      fprintf(stderr, "Allowing 'nobody' access to shell/id\n" );
      return 0;
    }
  }

  fprintf(stderr, "Denied\n" );
  return -1;
}

Although the UIDs are hard-code it actually worked! Yay!

I updated the code to convert the UID to a username, then check executables via the file /etc/can-exec/$USERNAME.conf, and this also worked.

I don't expect anybody to actually use this code, but I do think I've reached a point where I can pretend I've written a useful (or non-pointless) LSM at last. That means I can stop.

| 1 comment.

 

Linux security modules, round two.

Sunday, 25 June 2017

So recently I wrote a Linux Security Module (LSM) which would deny execution of commands, unless an extended attribute existed upon the filesystem belonging to the executables.

The whitelist-LSM worked well, but it soon became apparent that it was a little pointless. Most security changes are pointless unless you define what you're defending against - your "threat model".

In my case it was written largely as a learning experience, but also because I figured it seemed like it could be useful. However it wasn't actually as useful because you soon realize that you have to whitelist too much:

  • The redis-server binary must be executable, to the redis-user, otherwise it won't run.
  • /usr/bin/git must be executable to the git user.

In short there comes a point where user alice must run executable blah. If alice can run it, then so can mallory. At which point you realize the exercise is not so useful.

Taking a step back I realized that what I wanted to to prevent was the execution of unknown/unexpected, and malicious binaries How do you identify known-good binaries? Well hashes & checksums are good. So for my second attempt I figured I'd not look for a mere "flag" on a binary, instead look for a valid hash.

Now my second LSM is invoked for every binary that is executed by a user:

  • When a binary is executed the sha1 hash is calculated of the files contents.
  • If that matches the value stored in an extended attribute the execution is permitted.
    • If the extended-attribute is missing, or the checksum doesn't match, then the execution is denied.

In practice this is the same behaviour as the previous LSM - a binary is either executable, because there is a good hash, or it is not, because it is missing or bogus. If somebody deploys a binary rootkit this will definitely stop it from executing, but of course there is a huge hole - scripting-languages:

  • If /usr/bin/perl is whitelisted then /usr/bin/perl /tmp/exploit.pl will succeed.
  • If /usr/bin/python is whitelisted then the same applies.

Despite that the project was worthwhile, I can clearly describe what it is designed to achieve ("Deny the execution of unknown binaries", and "Deny binaries that have been modified"), and I learned how to hash a file from kernel-space - which was surprisingly simple.

(Yes I know about IMA and EVM - this was a simple project for learning purposes. Public-key signatures will be something I'll look at next/soon/later. :)

Perhaps the only other thing to explore is the complexity in allowing/denying actions based on the user - in a human-readable fashion, not via UIDs. So www-data can execute some programs, alice can run a different set of binaries, and git can only run /usr/bin/git.

Of course down that path lies apparmour, selinux, and madness..

| 2 comments.

 

So I accidentally wrote a linux security module

Friday, 2 June 2017

Tonight I read this weeks LWN quotes-page a little later than usual because I was busy at work for most of the day. Anyway as always LWNs content was awesome, and this particular list lead to an interesting discussion about a new Linux-Security-Module (LSM).

That read weirdly, what I was trying to say was that every Thursday morning I like to read LWN at work. Tonight is the first chance I had to get round to it.

One of the later replies in the thread was particularly interesting as it said:

Suggestion:

Create an security module that looks for the attribute

    security.WHITELISTED

on things being executed/mmapped and denys it if the attribute
isn't present. Create a program (whitelistd) that reads
/etc/whitelist.conf and scans the system to ensure that only
things on the list have the attribute.

So I figured that was a simple idea, and it didn't seem too hard even for myself as a non-kernel non-developer. There are several linux security modules included in the kernel-releases, beneath the top-level security/ directory, so I assumed I could copy & paste code around them to get something working.

During the course of all this work, which took about 90 minutes from start to Finnish (that pun never gets old), this online documentation was enormously useful:

Brief attr primer

If you're not familiar with the attr tool it's pretty simple. You can assign values to arbitrary labels on files. The only annoying thing is you have to use extra-flags to commands like rsync, tar, cp, etc, to preserve the damn things.

Set three attributes on the file named moi:

$ touch moi
$ attr -s forename -V "Steve"      moi
$ attr -s surname  -V "Kemp"       moi
$ attr -s name     -V "Steve Kemp" moi

Now list the attributes present:

$ attr -l moi
Attribute "name" has a 10 byte value for moi
Attribute "forename" has a 5 byte value for moi
Attribute "surname" has a 4 byte value for moi

And retrieve one?

$ attr -q -g name moi
Steve Kemp

LSM Skeleton

My initial starting point was to create "steve_lsm.c", with the following contents:

 #include <linux/lsm_hooks.h>

 /*
  * Log things for the moment.
  */
 static int steve_bprm_check_security(struct linux_binprm *bprm)
 {
     printk(KERN_INFO "STEVE LSM check of %s\n", bprm->filename);
     return 0;
 }

 /*
  * Only check exec().
  */
 static struct security_hook_list steve_hooks[] = {
     LSM_HOOK_INIT(bprm_check_security, steve_bprm_check_security),
 };

 /*
  * Somebody set us up the bomb.
  */
 static void __init steve_init(void)
 {
     security_add_hooks(steve_hooks, ARRAY_SIZE(steve_hooks), "steve");
     printk(KERN_INFO "STEVE LSM initialized\n");
 }

With that in place I had to modify the various KBuild files beneath security/ to make sure this could be selected as an LSM, and add in a Makefile to the new directory security/steve/.

With the boiler-plate done though, and the host machine rebooted into my new kernel it was simple to test things out.

Obviously the first step, post-boot, is to make sure that the module is active, which can be done in two ways, looking at the output of dmesg, and explicitly listing the modules available:

 ~# dmesg | grep STEVE | head -n2
 STEVE LSM initialized
 STEVE LSM check of /init

 $ echo $(cat /sys/kernel/security/lsm)
 capability,steve

Making the LSM functional

The next step was to make the module do more than mere logging. In short this is what we want:

  • If a binary is invoked by root - allow it.
    • Although note that this might leave a hole, if the user can enter a new namespace where their UID is 0..
  • If a binary is invoked by a non-root user look for an extended attribute on the target-file named security.WHITELISTED.
    • If this is present we allow the execution.
    • If this is missing we deny the execution.

NOTE we don't care what the content of the extended attribute is, we just care whether it exists or not.

Reading the extended attribute is pretty simple, using the __vfs_getxattr function. All in all our module becomes this:

  #include <linux/xattr.h>
  #include <linux/binfmts.h>
  #include <linux/lsm_hooks.h>
  #include <linux/sysctl.h>
  #include <linux/ptrace.h>
  #include <linux/prctl.h>
  #include <linux/ratelimit.h>
  #include <linux/workqueue.h>
  #include <linux/string_helpers.h>
  #include <linux/task_work.h>
  #include <linux/sched.h>
  #include <linux/spinlock.h>
  #include <linux/lsm_hooks.h>


  /*
   * Perform a check of a program execution/map.
   *
   * Return 0 if it should be allowed, -EPERM on block.
   */
  static int steve_bprm_check_security(struct linux_binprm *bprm)
  {
         // The current task & the UID it is running as.
         const struct task_struct *task = current;
         kuid_t uid = task->cred->uid;

         // The target we're checking
         struct dentry *dentry = bprm->file->f_path.dentry;
         struct inode *inode = d_backing_inode(dentry);

         // The size of the label-value (if any).
         int size = 0;

         // Root can access everything.
         if ( uid.val == 0 )
            return 0;

         size = __vfs_getxattr(dentry, inode, "user.whitelisted", NULL, 0);
         if ( size >= 0 )
         {
             printk(KERN_INFO "STEVE LSM check of %s resulted in %d bytes from 'user.whitelisted' - permitting access for UID %d\n", bprm->filename, size, uid.val );
             return 0;
         }

         printk(KERN_INFO "STEVE LSM check of %s denying access for UID %d [ERRO:%d] \n", bprm->filename, uid.val, size );
         return -EPERM;
  }

  /*
   * The hooks we wish to be installed.
   */
  static struct security_hook_list steve_hooks[] = {
       LSM_HOOK_INIT(bprm_check_security, steve_bprm_check_security),
  };

  /*
   * Initialize our module.
   */
  void __init steve_add_hooks(void)
  {
       /* register ourselves with the security framework */
       security_add_hooks(steve_hooks, ARRAY_SIZE(steve_hooks), "steve");

       printk(KERN_INFO "STEVE LSM initialized\n");
  }

Once again we reboot with this new kernel, and we test that the LSM is active. After the basic testing, as before, we can now test real functionality. By default no binaries will have the attribute we look for present - so we'd expect ALL commands to fail, unless executed by root. Let us test that:

~# su - nobody -s /bin/sh
No directory, logging in with HOME=/
Cannot execute /bin/sh: Operation not permitted

That looks like it worked. Let us allow users to run /bin/sh:

 ~# attr -s whitelisted -V 1 /bin/sh

Unfortunately that fails, because symlinks are weird, but repeating the test with /bin/dash works as expected:

 ~# su - nobody -s /bin/dash
 No directory, logging in with HOME=/
 Cannot execute /bin/dash: Operation not permitted

 ~# attr -s whitelisted -V 1 /bin/dash
 ~# attr -s whitelisted -V 1 /usr/bin/id

 ~# su - nobody -s /bin/dash
 No directory, logging in with HOME=/
 $ id
 uid=65534(nobody) gid=65534(nogroup) groups=65534(nogroup)

 $ uptime
 -su: 2: uptime: Operation not permitted

And our logging shows the useful results as we'd expect:

  STEVE LSM check of /usr/bin/id resulted in 1 bytes from 'user.WHITELISTED' - permitting access for UID 65534
  STEVE LSM check of /usr/bin/uptime denying access for UID 65534 [ERRO:-95]

Surprises

If you were paying careful attention you'll see that we changed what we did part-way through this guide.

  • The initial suggestion said to look for security.WHITELISTED.
  • But in the kernel module I look for user.whitelisted.
    • And when setting the attribute I only set whitelisted.

Not sure what is going on there, but it was very confusing. It appears to be the case that when you set an attribute a secret user. prefix is added to the name.

Could be worth some research by somebody with more time on their hands than I have.

Anyway I don't expect this is a terribly useful module, but it was my first, and I think it should be pretty stable. Feedback on my code certainly welcome!

| 3 comments.

 

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