[PATCH 13/15] selftests/seccomp: powerpc: Set syscall return during ptrace syscall exit

[Kees Cook]

Some archs (like ppc) only support changing the return code during
syscall exit when ptrace is used. As the syscall number might not
be available anymore during syscall exit, it needs to be saved
during syscall enter. Adjust the ptrace tests to do this.

Reported-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
Suggested-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
Link: https://lore.kernel.org/linux-kselftest/20200911181012.171027-1-cascardo@canonical.com/
Fixes: 58d0a862f573 ("seccomp: add tests for ptrace hole")
Signed-off-by: Kees Cook <keescook at chromium.org>
---
 tools/testing/selftests/seccomp/seccomp_bpf.c | 34 +++++++++++--------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index bbab2420d708..26c712c6a575 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -1949,12 +1949,19 @@ void tracer_seccomp(struct __test_metadata *_metadata, pid_t tracee,
 
 }
 
+FIXTURE(TRACE_syscall) {
+	struct sock_fprog prog;
+	pid_t tracer, mytid, mypid, parent;
+	long syscall_nr;
+};
+
 void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
 		   int status, void *args)
 {
-	int ret, nr;
+	int ret;
 	unsigned long msg;
 	static bool entry;
+	FIXTURE_DATA(TRACE_syscall) *self = args;
 
 	/*
 	 * The traditional way to tell PTRACE_SYSCALL entry/exit
@@ -1968,24 +1975,23 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
 	EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
 			: PTRACE_EVENTMSG_SYSCALL_EXIT, msg);
 
-	if (!entry)
-		return;
-
-	nr = get_syscall(_metadata, tracee);
+	/*
+	 * Some architectures only support setting return values during
+	 * syscall exit under ptrace, and on exit the syscall number may
+	 * no longer be available. Therefore, save it here, and call
+	 * "change syscall and set return values" on both entry and exit.
+	 */
+	if (entry)
+		self->syscall_nr = get_syscall(_metadata, tracee);
 
-	if (nr == __NR_getpid)
+	if (self->syscall_nr == __NR_getpid)
 		change_syscall(_metadata, tracee, __NR_getppid, 0);
-	if (nr == __NR_gettid)
+	if (self->syscall_nr == __NR_gettid)
 		change_syscall(_metadata, tracee, -1, 45000);
-	if (nr == __NR_openat)
+	if (self->syscall_nr == __NR_openat)
 		change_syscall(_metadata, tracee, -1, -ESRCH);
 }
 
-FIXTURE(TRACE_syscall) {
-	struct sock_fprog prog;
-	pid_t tracer, mytid, mypid, parent;
-};
-
 FIXTURE_VARIANT(TRACE_syscall) {
 	/*
 	 * All of the SECCOMP_RET_TRACE behaviors can be tested with either
@@ -2044,7 +2050,7 @@ FIXTURE_SETUP(TRACE_syscall)
 	self->tracer = setup_trace_fixture(_metadata,
 					   variant->use_ptrace ? tracer_ptrace
 							       : tracer_seccomp,
-					   NULL, variant->use_ptrace);
+					   self, variant->use_ptrace);
 
 	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
 	ASSERT_EQ(0, ret);
-- 
2.25.1