Add a sandbox for Python interpreter

Switch to user "nobody" and set additional limits.

3 files changed, 166 insertions, 0 deletions

diff --git a/python/runner/interpreter.py b/python/runner/interpreter.py
new file mode 100755
index 0000000..dae4d59
--- /dev/null
+++ b/python/runner/interpreter.py
@@ -0,0 +1,87 @@
+#!/usr/bin/python3
+
+import code
+import sys
+
+import seccomp
+
+f = seccomp.SyscallFilter(defaction=seccomp.KILL)
+# Necessary for Python.
+f.add_rule(seccomp.ALLOW, "brk")
+f.add_rule(seccomp.ALLOW, "exit_group")
+f.add_rule(seccomp.ALLOW, "ioctl")
+f.add_rule(seccomp.ALLOW, "mmap")
+f.add_rule(seccomp.ALLOW, "munmap")
+f.add_rule(seccomp.ALLOW, "rt_sigaction")
+f.add_rule(seccomp.ALLOW, "rt_sigreturn")
+
+# Mostly harmless.
+f.add_rule(seccomp.ALLOW, "mprotect")
+
+# Allow reading from stdin and writing to stdout/stderr.
+f.add_rule(seccomp.ALLOW, "read", seccomp.Arg(0, seccomp.EQ, sys.stdin.fileno()))
+f.add_rule(seccomp.ALLOW, "write", seccomp.Arg(0, seccomp.EQ, sys.stdout.fileno()))
+f.add_rule(seccomp.ALLOW, "write", seccomp.Arg(0, seccomp.EQ, sys.stderr.fileno()))
+
+# Needed for finding source code for exceptions.
+f.add_rule(seccomp.ALLOW, "stat")
+f.add_rule(seccomp.ALLOW, "open", seccomp.Arg(1, seccomp.MASKED_EQ, 0x3, 0))  # O_RDONLY
+f.add_rule(seccomp.ALLOW, "close")
+f.add_rule(seccomp.ALLOW, "read")
+f.add_rule(seccomp.ALLOW, "fstat")
+f.add_rule(seccomp.ALLOW, "lseek")
+f.add_rule(seccomp.ALLOW, "fcntl")
+
+# Needed for help().
+f.add_rule(seccomp.ALLOW, "openat", seccomp.Arg(2, seccomp.MASKED_EQ, 0x3, 0))  # O_RDONLY
+f.add_rule(seccomp.ALLOW, "getdents")
+f.add_rule(seccomp.ALLOW, "getrlimit", seccomp.Arg(0, seccomp.EQ, 3))  # RLIMIT_STACK
+f.add_rule(seccomp.ALLOW, "getrlimit", seccomp.Arg(0, seccomp.EQ, 7))  # RLIMIT_NOFILE
+
+# Needed for code.InteractiveConsole.
+f.add_rule(seccomp.ALLOW, "access")
+f.add_rule(seccomp.ALLOW, "select")
+f.load()
+
+class MyConsole(code.InteractiveConsole):
+    def interact(self, banner=None):
+        if banner is not None:
+            self.write('{}\n'.format(banner))
+
+        buffer = []
+        prompt = '>>> '
+        while True:
+            try:
+                line = input(prompt)
+                # Assume we are running the user's program; silence the prompt.
+                if line == 'exec("""\\':
+                    self.write('<run>\n')
+                    prompt = ''
+
+                buffer.append(line)
+                source = '\n'.join(buffer)
+                more = self.runsource(source)
+                if more:
+                    if prompt:
+                        prompt = '... '
+                else:
+                    prompt = '>>> '
+                    buffer = []
+            except KeyboardInterrupt:
+                prompt = '>>> '
+                buffer = []
+                self.write('\n')
+            except EOFError:
+                break
+
+    def runcode(self, code):
+        try:
+            exec(code, self.locals)
+        except KeyboardInterrupt:
+            # Don't show traceback on SIGINT.
+            self.write('^C')
+            raise
+        except:
+            self.showtraceback()
+
+MyConsole().interact()
diff --git a/python/runner/sandbox.c b/python/runner/sandbox.c
new file mode 100644
index 0000000..12e2720
--- /dev/null
+++ b/python/runner/sandbox.c
@@ -0,0 +1,47 @@
+#include <fcntl.h>
+#include <pwd.h>
+#include <stdio.h>
+#include <sys/prctl.h>
+#include <sys/resource.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+int main(int argc, char* argv[])
+{
+	if (argc < 3) {
+		fprintf(stderr, "usage: %s USERNAME FILE\n", argv[0]);
+		return 1;
+	}
+
+	// switch user (requires root or "setcap cap_setuid,cap_setgid+ep")
+	char const* username = argv[1];
+	struct passwd const* pw = getpwnam(username);
+	if (!pw) {
+		fprintf(stderr, "no such user: %s\n", username);
+		return 1;
+	}
+	int ret = 0;
+	if ((ret = setuid(pw->pw_uid)) != 0)
+		fprintf(stderr, "setuid returned %d\n", ret);
+	if ((ret = setgid(pw->pw_gid)) != 0)
+		fprintf(stderr, "setgid returned %d\n", ret);
+
+	// limit CPU time to 1 second
+	struct rlimit const cpu_limit = { .rlim_cur = 1, .rlim_max = 1 };
+	if ((ret = setrlimit(RLIMIT_CPU, &cpu_limit)) != 0)
+		fprintf(stderr, "setrlimit(CPU) returned %d\n", ret);
+
+	// don't allow writing files of any size
+	struct rlimit const fsize_limit = { .rlim_cur = 0, .rlim_max = 0 };
+	if ((ret = setrlimit(RLIMIT_FSIZE, &fsize_limit)) != 0)
+		fprintf(stderr, "setrlimit(FSIZE) returned %d\n", ret);
+
+	// there will be no fork
+	struct rlimit const nproc_limit = { .rlim_cur = 0, .rlim_max = 0 };
+	if ((ret = setrlimit(RLIMIT_NPROC, &nproc_limit)) != 0)
+		fprintf(stderr, "setrlimit(NPROC) returned %d\n", ret);
+
+	char* const args[] = { argv[2], (char*)0 };
+	return execvp(argv[2], args);
+}
diff --git a/python/runner/terminator.c b/python/runner/terminator.c
new file mode 100644
index 0000000..a994bde
--- /dev/null
+++ b/python/runner/terminator.c
@@ -0,0 +1,32 @@
+#include <pwd.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+int main(int argc, char* argv[])
+{
+	if (argc < 4) {
+		fprintf(stderr, "usage: %s USERNAME PID SIGNAL\n", argv[0]);
+		return 1;
+	}
+
+	// switch user (requires root or "setcap cap_setuid,cap_setgid+ep")
+	char const* username = argv[1];
+	struct passwd const* pw = getpwnam(username);
+	if (!pw) {
+		fprintf(stderr, "no such user: %s\n", username);
+		return 1;
+	}
+	int ret = 0;
+	if ((ret = setuid(pw->pw_uid)) != 0)
+		fprintf(stderr, "setuid returned %d\n", ret);
+	if ((ret = setgid(pw->pw_gid)) != 0)
+		fprintf(stderr, "setgid returned %d\n", ret);
+
+	pid_t pid = atol(argv[2]);
+	int signum = atoi(argv[3]);
+	kill(pid, signum);
+	return 0;
+}