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Commit 58d0a862 authored by Kees Cook's avatar Kees Cook
Browse files

seccomp: add tests for ptrace hole 



One problem with seccomp was that ptrace could be used to change a
syscall after seccomp filtering had completed. This was a well documented
limitation, and it was recommended to block ptrace when defining a filter
to avoid this problem. This can be quite a limitation for containers or
other places where ptrace is desired even under seccomp filters.

This adds tests for both SECCOMP_RET_TRACE and PTRACE_SYSCALL manipulations.

Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@kernel.org>
parent 40d27378

tools/testing/selftests/seccomp/seccomp_bpf.c

+165 −11
Original line number Diff line number Diff line
@@ -1021,8 +1021,8 @@ void tracer_stop(int sig) 
typedef void tracer_func_t(struct __test_metadata *_metadata,

			   pid_t tracee, int status, void *args);



void tracer(struct __test_metadata *_metadata, int fd, pid_t tracee,

	    tracer_func_t tracer_func, void *args)

void start_tracer(struct __test_metadata *_metadata, int fd, pid_t tracee,

	    tracer_func_t tracer_func, void *args, bool ptrace_syscall)

{

	int ret = -1;

	struct sigaction action = {
@@ -1042,12 +1042,16 @@ void tracer(struct __test_metadata *_metadata, int fd, pid_t tracee, 
	/* Wait for attach stop */

	wait(NULL);



	ret = ptrace(PTRACE_SETOPTIONS, tracee, NULL, PTRACE_O_TRACESECCOMP);

	ret = ptrace(PTRACE_SETOPTIONS, tracee, NULL, ptrace_syscall ?

						      PTRACE_O_TRACESYSGOOD :

						      PTRACE_O_TRACESECCOMP);

	ASSERT_EQ(0, ret) {

		TH_LOG("Failed to set PTRACE_O_TRACESECCOMP");

		kill(tracee, SIGKILL);

	}

	ptrace(PTRACE_CONT, tracee, NULL, 0);

	ret = ptrace(ptrace_syscall ? PTRACE_SYSCALL : PTRACE_CONT,

		     tracee, NULL, 0);

	ASSERT_EQ(0, ret);



	/* Unblock the tracee */

	ASSERT_EQ(1, write(fd, "A", 1));
@@ -1063,12 +1067,13 @@ void tracer(struct __test_metadata *_metadata, int fd, pid_t tracee, 
			/* Child is dead. Time to go. */

			return;



		/* Make sure this is a seccomp event. */

		ASSERT_EQ(true, IS_SECCOMP_EVENT(status));

		/* Check if this is a seccomp event. */

		ASSERT_EQ(!ptrace_syscall, IS_SECCOMP_EVENT(status));



		tracer_func(_metadata, tracee, status, args);



		ret = ptrace(PTRACE_CONT, tracee, NULL, NULL);

		ret = ptrace(ptrace_syscall ? PTRACE_SYSCALL : PTRACE_CONT,

			     tracee, NULL, 0);

		ASSERT_EQ(0, ret);

	}

	/* Directly report the status of our test harness results. */
@@ -1079,7 +1084,7 @@ void tracer(struct __test_metadata *_metadata, int fd, pid_t tracee, 
void cont_handler(int num)

{ }

pid_t setup_trace_fixture(struct __test_metadata *_metadata,

			  tracer_func_t func, void *args)

			  tracer_func_t func, void *args, bool ptrace_syscall)

{

	char sync;

	int pipefd[2];
@@ -1095,7 +1100,8 @@ pid_t setup_trace_fixture(struct __test_metadata *_metadata, 
	signal(SIGALRM, cont_handler);

	if (tracer_pid == 0) {

		close(pipefd[0]);

		tracer(_metadata, pipefd[1], tracee, func, args);

		start_tracer(_metadata, pipefd[1], tracee, func, args,

			     ptrace_syscall);

		syscall(__NR_exit, 0);

	}

	close(pipefd[1]);
@@ -1177,7 +1183,7 @@ FIXTURE_SETUP(TRACE_poke) 


	/* Launch tracer. */

	self->tracer = setup_trace_fixture(_metadata, tracer_poke,

					   &self->tracer_args);

					   &self->tracer_args, false);

}



FIXTURE_TEARDOWN(TRACE_poke)
@@ -1399,6 +1405,29 @@ void tracer_syscall(struct __test_metadata *_metadata, pid_t tracee, 


}



void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,

		   int status, void *args)

{

	int ret, nr;

	unsigned long msg;

	static bool entry;



	/* Make sure we got an empty message. */

	ret = ptrace(PTRACE_GETEVENTMSG, tracee, NULL, &msg);

	EXPECT_EQ(0, ret);

	EXPECT_EQ(0, msg);



	/* The only way to tell PTRACE_SYSCALL entry/exit is by counting. */

	entry = !entry;

	if (!entry)

		return;



	nr = get_syscall(_metadata, tracee);



	if (nr == __NR_getpid)

		change_syscall(_metadata, tracee, __NR_getppid);

}



FIXTURE_DATA(TRACE_syscall) {

	struct sock_fprog prog;

	pid_t tracer, mytid, mypid, parent;
@@ -1440,7 +1469,8 @@ FIXTURE_SETUP(TRACE_syscall) 
	ASSERT_NE(self->parent, self->mypid);



	/* Launch tracer. */

	self->tracer = setup_trace_fixture(_metadata, tracer_syscall, NULL);

	self->tracer = setup_trace_fixture(_metadata, tracer_syscall, NULL,

					   false);

}



FIXTURE_TEARDOWN(TRACE_syscall)
@@ -1500,6 +1530,130 @@ TEST_F(TRACE_syscall, syscall_dropped) 
	EXPECT_NE(self->mytid, syscall(__NR_gettid));

}



TEST_F(TRACE_syscall, skip_after_RET_TRACE)

{

	struct sock_filter filter[] = {

		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,

			offsetof(struct seccomp_data, nr)),

		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getppid, 0, 1),

		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ERRNO | EPERM),

		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),

	};

	struct sock_fprog prog = {

		.len = (unsigned short)ARRAY_SIZE(filter),

		.filter = filter,

	};

	long ret;



	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);

	ASSERT_EQ(0, ret);



	/* Install fixture filter. */

	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->prog, 0, 0);

	ASSERT_EQ(0, ret);



	/* Install "errno on getppid" filter. */

	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog, 0, 0);

	ASSERT_EQ(0, ret);



	/* Tracer will redirect getpid to getppid, and we should see EPERM. */

	EXPECT_EQ(-1, syscall(__NR_getpid));

	EXPECT_EQ(EPERM, errno);

}



TEST_F_SIGNAL(TRACE_syscall, kill_after_RET_TRACE, SIGSYS)

{

	struct sock_filter filter[] = {

		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,

			offsetof(struct seccomp_data, nr)),

		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getppid, 0, 1),

		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_KILL),

		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),

	};

	struct sock_fprog prog = {

		.len = (unsigned short)ARRAY_SIZE(filter),

		.filter = filter,

	};

	long ret;



	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);

	ASSERT_EQ(0, ret);



	/* Install fixture filter. */

	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->prog, 0, 0);

	ASSERT_EQ(0, ret);



	/* Install "death on getppid" filter. */

	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog, 0, 0);

	ASSERT_EQ(0, ret);



	/* Tracer will redirect getpid to getppid, and we should die. */

	EXPECT_NE(self->mypid, syscall(__NR_getpid));

}



TEST_F(TRACE_syscall, skip_after_ptrace)

{

	struct sock_filter filter[] = {

		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,

			offsetof(struct seccomp_data, nr)),

		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getppid, 0, 1),

		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ERRNO | EPERM),

		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),

	};

	struct sock_fprog prog = {

		.len = (unsigned short)ARRAY_SIZE(filter),

		.filter = filter,

	};

	long ret;



	/* Swap SECCOMP_RET_TRACE tracer for PTRACE_SYSCALL tracer. */

	teardown_trace_fixture(_metadata, self->tracer);

	self->tracer = setup_trace_fixture(_metadata, tracer_ptrace, NULL,

					   true);



	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);

	ASSERT_EQ(0, ret);



	/* Install "errno on getppid" filter. */

	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog, 0, 0);

	ASSERT_EQ(0, ret);



	/* Tracer will redirect getpid to getppid, and we should see EPERM. */

	EXPECT_EQ(-1, syscall(__NR_getpid));

	EXPECT_EQ(EPERM, errno);

}



TEST_F_SIGNAL(TRACE_syscall, kill_after_ptrace, SIGSYS)

{

	struct sock_filter filter[] = {

		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,

			offsetof(struct seccomp_data, nr)),

		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getppid, 0, 1),

		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_KILL),

		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),

	};

	struct sock_fprog prog = {

		.len = (unsigned short)ARRAY_SIZE(filter),

		.filter = filter,

	};

	long ret;



	/* Swap SECCOMP_RET_TRACE tracer for PTRACE_SYSCALL tracer. */

	teardown_trace_fixture(_metadata, self->tracer);

	self->tracer = setup_trace_fixture(_metadata, tracer_ptrace, NULL,

					   true);



	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);

	ASSERT_EQ(0, ret);



	/* Install "death on getppid" filter. */

	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog, 0, 0);

	ASSERT_EQ(0, ret);



	/* Tracer will redirect getpid to getppid, and we should die. */

	EXPECT_NE(self->mypid, syscall(__NR_getpid));

}



#ifndef __NR_seccomp

# if defined(__i386__)

#  define __NR_seccomp 354