[PATCH] ptrace: fix task_join_group_stop() for the case when current is traced

From: Oleg Nesterov
Date: Mon Oct 19 2020 - 09:42:52 EST


This testcase

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <pthread.h>
#include <assert.h>

void *tf(void *arg)
{
return NULL;
}

int main(void)
{
int pid = fork();
if (!pid) {
kill(getpid(), SIGSTOP);

pthread_t th;
pthread_create(&th, NULL, tf, NULL);

return 0;
}

waitpid(pid, NULL, WSTOPPED);

ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_TRACECLONE);
waitpid(pid, NULL, 0);

ptrace(PTRACE_CONT, pid, 0,0);
waitpid(pid, NULL, 0);

int status;
int thread = waitpid(-1, &status, 0);
assert(thread > 0 && thread != pid);
assert(status == 0x80137f);

return 0;
}

fails and triggers WARN_ON_ONCE(!signr) in do_jobctl_trap().

This is because task_join_group_stop() has 2 problems when current is traced:

1. We can't rely on the "JOBCTL_STOP_PENDING" check, a stopped tracee
can be woken up by debugger and it can clone another thread which
should join the group-stop.

We need to check group_stop_count || SIGNAL_STOP_STOPPED.

2. If SIGNAL_STOP_STOPPED is already set, we should not increment
sig->group_stop_count and add JOBCTL_STOP_CONSUME. The new thread
should stop without another do_notify_parent_cldstop() report.

To clarify, the problem is very old and we should blame ptrace_init_task().
But now that we have task_join_group_stop() it makes more sense to fix this
helper to avoid the code duplication.

Reported-by: syzbot+3485e3773f7da290eecc@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
kernel/signal.c | 19 ++++++++++---------
1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index a38b3edc6851..ef8f2a28d37c 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -391,16 +391,17 @@ static bool task_participate_group_stop(struct task_struct *task)

void task_join_group_stop(struct task_struct *task)
{
+ unsigned long mask = current->jobctl & JOBCTL_STOP_SIGMASK;
+ struct signal_struct *sig = current->signal;
+
+ if (sig->group_stop_count) {
+ sig->group_stop_count++;
+ mask |= JOBCTL_STOP_CONSUME;
+ } else if (!(sig->flags & SIGNAL_STOP_STOPPED))
+ return;
+
/* Have the new thread join an on-going signal group stop */
- unsigned long jobctl = current->jobctl;
- if (jobctl & JOBCTL_STOP_PENDING) {
- struct signal_struct *sig = current->signal;
- unsigned long signr = jobctl & JOBCTL_STOP_SIGMASK;
- unsigned long gstop = JOBCTL_STOP_PENDING | JOBCTL_STOP_CONSUME;
- if (task_set_jobctl_pending(task, signr | gstop)) {
- sig->group_stop_count++;
- }
- }
+ task_set_jobctl_pending(task, mask | JOBCTL_STOP_PENDING);
}

/*
--
2.25.1.362.g51ebf55