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

Michael Ellerman
2020-09-14T05:47:20Z

Kees Cook <keescook@chromium.org> writes:
> 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.

I'm not that across all the fixture stuff, but if I'm reading it right
you're now calling change_syscall() on both entry and exit for all
arches.

That should work, but it no longer tests changing the return code on
entry on the arches that support it, which seems like a backward step?

cheers

> Reported-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
> Suggested-by: Thadeu Lima de Souza Cascardo <cascardo@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@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