Subject: Re: [PATCH] selftests/seccomp: fix ptrace tests on powerpc

From: Kees Cook <[email-masked]>
Date: Tue, 8 Sep 2020 16:18:17 -0700

On Tue, Jun 30, 2020 at 01:47:39PM -0300, Thadeu Lima de Souza Cascardo wrote:
> As pointed out by Michael Ellerman, the ptrace ABI on powerpc does not
> allow or require the return code to be set on syscall entry when
> skipping the syscall. It will always return ENOSYS and the return code
> must be set on syscall exit.
> 
> This code does that, behaving more similarly to strace. It still sets
> the return code on entry, which is overridden on powerpc, and it will
> always repeat the same on exit. Also, on powerpc, the errno is not
> inverted, and depends on ccr.so being set.
> 
> This has been tested on powerpc and amd64.
> 
> Cc: Michael Ellerman <[email-masked]>
> Cc: Kees Cook <[email-masked]>
> Signed-off-by: Thadeu Lima de Souza Cascardo <[email-masked]>

Yikes, I missed this from a while ago. I apologize for responding so
late!

This appears still unfixed; is that correct?

> ---
>  tools/testing/selftests/seccomp/seccomp_bpf.c | 24 +++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index 252140a52553..b90a9190ba88 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -1738,6 +1738,14 @@ void change_syscall(struct __test_metadata *_metadata,
>  		TH_LOG("Can't modify syscall return on this architecture");
>  #else
>  		regs.SYSCALL_RET = result;
> +# if defined(__powerpc__)
> +		if (result < 0) {
> +			regs.SYSCALL_RET = -result;
> +			regs.ccr |= 0x10000000;
> +		} else {
> +			regs.ccr &= ~0x10000000;
> +		}
> +# endif
>  #endif

Just so I understand correctly: for ppc to "see" this result, it needs
to be both negative AND have this specific register set?

>  
>  #ifdef HAVE_GETREGS
> @@ -1796,6 +1804,7 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
>  	int ret, nr;
>  	unsigned long msg;
>  	static bool entry;
> +	int *syscall_nr = args;
>  
>  	/*
>  	 * The traditional way to tell PTRACE_SYSCALL entry/exit
> @@ -1809,10 +1818,15 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
>  	EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
>  			: PTRACE_EVENTMSG_SYSCALL_EXIT, msg);
>  
> -	if (!entry)
> +	if (!entry && !syscall_nr)
>  		return;
>  
> -	nr = get_syscall(_metadata, tracee);
> +	if (entry)
> +		nr = get_syscall(_metadata, tracee);
> +	else
> +		nr = *syscall_nr;

This is weird? Shouldn't get_syscall() be modified to do the right thing
here instead of depending on the extra arg?

> +	if (syscall_nr)
> +		*syscall_nr = nr;
>  
>  	if (nr == __NR_getpid)
>  		change_syscall(_metadata, tracee, __NR_getppid, 0);
> @@ -1889,9 +1903,10 @@ TEST_F(TRACE_syscall, ptrace_syscall_redirected)
>  
>  TEST_F(TRACE_syscall, ptrace_syscall_errno)
>  {
> +	int syscall_nr = -1;
>  	/* Swap SECCOMP_RET_TRACE tracer for PTRACE_SYSCALL tracer. */
>  	teardown_trace_fixture(_metadata, self->tracer);
> -	self->tracer = setup_trace_fixture(_metadata, tracer_ptrace, NULL,
> +	self->tracer = setup_trace_fixture(_metadata, tracer_ptrace, &syscall_nr,
>  					   true);
>  
>  	/* Tracer should skip the open syscall, resulting in ESRCH. */
> @@ -1900,9 +1915,10 @@ TEST_F(TRACE_syscall, ptrace_syscall_errno)
>  
>  TEST_F(TRACE_syscall, ptrace_syscall_faked)
>  {
> +	int syscall_nr = -1;
>  	/* Swap SECCOMP_RET_TRACE tracer for PTRACE_SYSCALL tracer. */
>  	teardown_trace_fixture(_metadata, self->tracer);
> -	self->tracer = setup_trace_fixture(_metadata, tracer_ptrace, NULL,
> +	self->tracer = setup_trace_fixture(_metadata, tracer_ptrace, &syscall_nr,
>  					   true);
>  
>  	/* Tracer should skip the gettid syscall, resulting fake pid. */
> -- 
> 2.25.1
> 

-- 
Kees Cook