From: Oleg Nesterov <oleg@redhat.com> commit 7b3c36fc4c231ca532120bbc0df67a12f09c1d96 upstream. 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@syzkaller.appspotmail.com Signed-off-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Cc: Jens Axboe <axboe@kernel.dk> Cc: Christian Brauner <christian@brauner.io> Cc: "Eric W . Biederman" <ebiederm@xmission.com> Cc: Zhiqiang Liu <liuzhiqiang26@huawei.com> Cc: Tejun Heo <tj@kernel.org> Cc: <stable@vger.kernel.org> Link: https://lkml.kernel.org/r/20201019134237.GA18810@redhat.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> --- kernel/signal.c | 19 ++++++++++--------- 1 file changed, 10 insertions(+), 9 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index 091a3a313410..a39ca8841c8f 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -387,16 +387,17 @@ 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