selftests/seccomp: Enhance per-arch ptrace syscall skip tests

Passing EPERM during syscall skipping was confusing since the test wasn't
actually exercising the errno evaluation -- it was just passing a literal
"1" (EPERM). Instead, expand the tests to check both direct value returns
(positive, 45000 in this case), and errno values (negative, -ESRCH in this
case) to check both fake success and fake failure during syscall skipping.
Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: a33b2d03 ("selftests/seccomp: Add tests for basic ptrace actions")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -1608,7 +1608,16 @@ TEST_F(TRACE_poke, getpid_runs_normally)
 #ifdef SYSCALL_NUM_RET_SHARE_REG
 # define EXPECT_SYSCALL_RETURN(val, action)	EXPECT_EQ(-1, action)
 #else
-# define EXPECT_SYSCALL_RETURN(val, action)	EXPECT_EQ(val, action)
+# define EXPECT_SYSCALL_RETURN(val, action)		\
+	do {						\
+		errno = 0;				\
+		if (val < 0) {				\
+			EXPECT_EQ(-1, action);		\
+			EXPECT_EQ(-(val), errno);	\
+		} else {				\
+			EXPECT_EQ(val, action);		\
+		}					\
+	} while (0)
 #endif
 
 /* Use PTRACE_GETREGS and PTRACE_SETREGS when available. This is useful for
@@ -1647,7 +1656,7 @@ int get_syscall(struct __test_metadata *_metadata, pid_t tracee)
 
 /* Architecture-specific syscall changing routine. */
 void change_syscall(struct __test_metadata *_metadata,
-		    pid_t tracee, int syscall)
+		    pid_t tracee, int syscall, int result)
 {
 	int ret;
 	ARCH_REGS regs;
@@ -1706,7 +1715,7 @@ void change_syscall(struct __test_metadata *_metadata,
 #ifdef SYSCALL_NUM_RET_SHARE_REG
 		TH_LOG("Can't modify syscall return on this architecture");
 #else
-		regs.SYSCALL_RET = EPERM;
+		regs.SYSCALL_RET = result;
 #endif
 
 #ifdef HAVE_GETREGS
@@ -1734,14 +1743,19 @@ void tracer_syscall(struct __test_metadata *_metadata, pid_t tracee,
 	case 0x1002:
 		/* change getpid to getppid. */
 		EXPECT_EQ(__NR_getpid, get_syscall(_metadata, tracee));
-		change_syscall(_metadata, tracee, __NR_getppid);
+		change_syscall(_metadata, tracee, __NR_getppid, 0);
 		break;
 	case 0x1003:
-		/* skip gettid. */
+		/* skip gettid with valid return code. */
 		EXPECT_EQ(__NR_gettid, get_syscall(_metadata, tracee));
-		change_syscall(_metadata, tracee, -1);
+		change_syscall(_metadata, tracee, -1, 45000);
 		break;
 	case 0x1004:
+		/* skip openat with error. */
+		EXPECT_EQ(__NR_openat, get_syscall(_metadata, tracee));
+		change_syscall(_metadata, tracee, -1, -ESRCH);
+		break;
+	case 0x1005:
 		/* do nothing (allow getppid) */
 		EXPECT_EQ(__NR_getppid, get_syscall(_metadata, tracee));
 		break;
@@ -1774,9 +1788,11 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
 	nr = get_syscall(_metadata, tracee);
 
 	if (nr == __NR_getpid)
-		change_syscall(_metadata, tracee, __NR_getppid);
+		change_syscall(_metadata, tracee, __NR_getppid, 0);
+	if (nr == __NR_gettid)
+		change_syscall(_metadata, tracee, -1, 45000);
 	if (nr == __NR_openat)
-		change_syscall(_metadata, tracee, -1);
+		change_syscall(_metadata, tracee, -1, -ESRCH);
 }
 
 FIXTURE_DATA(TRACE_syscall) {
@@ -1793,8 +1809,10 @@ FIXTURE_SETUP(TRACE_syscall)
 		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_TRACE | 0x1002),
 		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_gettid, 0, 1),
 		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_TRACE | 0x1003),
-		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getppid, 0, 1),
+		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_openat, 0, 1),
 		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_TRACE | 0x1004),
+		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getppid, 0, 1),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_TRACE | 0x1005),
 		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
 	};
 
@@ -1842,15 +1860,26 @@ TEST_F(TRACE_syscall, ptrace_syscall_redirected)
 	EXPECT_NE(self->mypid, syscall(__NR_getpid));
 }
 
-TEST_F(TRACE_syscall, ptrace_syscall_dropped)
+TEST_F(TRACE_syscall, ptrace_syscall_errno)
+{
+	/* 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);
+
+	/* Tracer should skip the open syscall, resulting in ESRCH. */
+	EXPECT_SYSCALL_RETURN(-ESRCH, syscall(__NR_openat));
+}
+
+TEST_F(TRACE_syscall, ptrace_syscall_faked)
 {
 	/* 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);
 
-	/* Tracer should skip the open syscall, resulting in EPERM. */
-	EXPECT_SYSCALL_RETURN(EPERM, syscall(__NR_openat));
+	/* Tracer should skip the gettid syscall, resulting fake pid. */
+	EXPECT_SYSCALL_RETURN(45000, syscall(__NR_gettid));
 }
 
 TEST_F(TRACE_syscall, syscall_allowed)
@@ -1883,7 +1912,21 @@ TEST_F(TRACE_syscall, syscall_redirected)
 	EXPECT_NE(self->mypid, syscall(__NR_getpid));
 }
 
-TEST_F(TRACE_syscall, syscall_dropped)
+TEST_F(TRACE_syscall, syscall_errno)
+{
+	long ret;
+
+	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+	ASSERT_EQ(0, ret);
+
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->prog, 0, 0);
+	ASSERT_EQ(0, ret);
+
+	/* openat has been skipped and an errno return. */
+	EXPECT_SYSCALL_RETURN(-ESRCH, syscall(__NR_openat));
+}
+
+TEST_F(TRACE_syscall, syscall_faked)
 {
 	long ret;
 
@@ -1894,8 +1937,7 @@ TEST_F(TRACE_syscall, syscall_dropped)
 	ASSERT_EQ(0, ret);
 
 	/* gettid has been skipped and an altered return value stored. */
-	EXPECT_SYSCALL_RETURN(EPERM, syscall(__NR_gettid));
-	EXPECT_NE(self->mytid, syscall(__NR_gettid));
+	EXPECT_SYSCALL_RETURN(45000, syscall(__NR_gettid));
 }
 
 TEST_F(TRACE_syscall, skip_after_RET_TRACE)