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: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@kernel.org>

1 files changed, 165 insertions, 11 deletions

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index 2e58549b2f02..03f1fa495d74 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -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