Re: [PATCH 2/4] selftests/ptrace: add test cases for dead-locks

Kees Cook
2020-03-10T21:36:27Z

On Tue, Mar 10, 2020 at 02:44:01PM +0100, Bernd Edlinger wrote:
> This adds test cases for ptrace deadlocks.
>
> Additionally fixes a compile problem in get_syscall_info.c,
> observed with gcc-4.8.4:
>
> get_syscall_info.c: In function 'get_syscall_info':
> get_syscall_info.c:93:3: error: 'for' loop initial declarations are only
> allowed in C99 mode
> for (unsigned int i = 0; i < ARRAY_SIZE(args); ++i) {
> ^
> get_syscall_info.c:93:3: note: use option -std=c99 or -std=gnu99 to compile
> your code

*discomfort noises* (see below)

>
> Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
> ---
> tools/testing/selftests/ptrace/Makefile | 4 +-
> tools/testing/selftests/ptrace/vmaccess.c | 86 +++++++++++++++++++++++++++++++
> 2 files changed, 88 insertions(+), 2 deletions(-)
> create mode 100644 tools/testing/selftests/ptrace/vmaccess.c
>
> diff --git a/tools/testing/selftests/ptrace/Makefile b/tools/testing/selftests/ptrace/Makefile
> index c0b7f89..2f1f532 100644
> --- a/tools/testing/selftests/ptrace/Makefile
> +++ b/tools/testing/selftests/ptrace/Makefile
> @@ -1,6 +1,6 @@
> # SPDX-License-Identifier: GPL-2.0-only
> -CFLAGS += -iquote../../../../include/uapi -Wall
> +CFLAGS += -std=c99 -pthread -iquote../../../../include/uapi -Wall

This isn't the common solution in the kernel (the variable declaration
would just be lifted out of the loop), but as it's selftest code, which
does lots of special things ... I *guess* this is okay.

>
> -TEST_GEN_PROGS := get_syscall_info peeksiginfo
> +TEST_GEN_PROGS := get_syscall_info peeksiginfo vmaccess

I love having this deadlock test added to the selftests.

I think I need to make an improvement to the test harness, though, as
the failure mode right now just blows up after the 30 second timeout
and leaves this deadlocked:

$ ./vmaccess
[==========] Running 2 tests from 1 test cases.
[ RUN ] global.vmaccess
Alarm clock
$ ps
PID TTY TIME CMD
2605 pts/0 00:00:00 bash
23360 pts/0 00:00:00 vmaccess
23361 pts/0 00:00:00 vmaccess
23363 pts/0 00:00:00 ps

But that's mostly unrelated to this code.

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

>
> include ../lib.mk
> diff --git a/tools/testing/selftests/ptrace/vmaccess.c b/tools/testing/selftests/ptrace/vmaccess.c
> new file mode 100644
> index 0000000..4db327b
> --- /dev/null
> +++ b/tools/testing/selftests/ptrace/vmaccess.c
> @@ -0,0 +1,86 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Copyright (c) 2020 Bernd Edlinger <bernd.edlinger@hotmail.de>
> + * All rights reserved.
> + *
> + * Check whether /proc/$pid/mem can be accessed without causing deadlocks
> + * when de_thread is blocked with ->cred_guard_mutex held.
> + */
> +
> +#include "../kselftest_harness.h"
> +#include <stdio.h>
> +#include <fcntl.h>
> +#include <pthread.h>
> +#include <signal.h>
> +#include <unistd.h>
> +#include <sys/ptrace.h>
> +
> +static void *thread(void *arg)
> +{
> + ptrace(PTRACE_TRACEME, 0, 0L, 0L);
> + return NULL;
> +}
> +
> +TEST(vmaccess)
> +{
> + int f, pid = fork();
> + char mm[64];
> +
> + if (!pid) {
> + pthread_t pt;
> +
> + pthread_create(&pt, NULL, thread, NULL);
> + pthread_join(pt, NULL);
> + execlp("true", "true", NULL);
> + }
> +
> + sleep(1);
> + sprintf(mm, "/proc/%d/mem", pid);
> + f = open(mm, O_RDONLY);
> + ASSERT_GE(f, 0);
> + close(f);
> + f = kill(pid, SIGCONT);
> + ASSERT_EQ(f, 0);
> +}
> +
> +TEST(attach)
> +{
> + int s, k, pid = fork();
> +
> + if (!pid) {
> + pthread_t pt;
> +
> + pthread_create(&pt, NULL, thread, NULL);
> + pthread_join(pt, NULL);
> + execlp("sleep", "sleep", "2", NULL);
> + }
> +
> + sleep(1);
> + k = ptrace(PTRACE_ATTACH, pid, 0L, 0L);
> + ASSERT_EQ(errno, EAGAIN);
> + ASSERT_EQ(k, -1);
> + k = waitpid(-1, &s, WNOHANG);
> + ASSERT_NE(k, -1);
> + ASSERT_NE(k, 0);
> + ASSERT_NE(k, pid);
> + ASSERT_EQ(WIFEXITED(s), 1);
> + ASSERT_EQ(WEXITSTATUS(s), 0);
> + sleep(1);
> + k = ptrace(PTRACE_ATTACH, pid, 0L, 0L);
> + ASSERT_EQ(k, 0);
> + k = waitpid(-1, &s, 0);
> + ASSERT_EQ(k, pid);
> + ASSERT_EQ(WIFSTOPPED(s), 1);
> + ASSERT_EQ(WSTOPSIG(s), SIGSTOP);
> + k = ptrace(PTRACE_DETACH, pid, 0L, 0L);
> + ASSERT_EQ(k, 0);
> + k = waitpid(-1, &s, 0);
> + ASSERT_EQ(k, pid);
> + ASSERT_EQ(WIFEXITED(s), 1);
> + ASSERT_EQ(WEXITSTATUS(s), 0);
> + k = waitpid(-1, NULL, 0);
> + ASSERT_EQ(k, -1);
> + ASSERT_EQ(errno, ECHILD);
> +}
> +
> +TEST_HARNESS_MAIN
> --
> 1.9.1

--
Kees Cook