Commit b72c1869 authored Apr 16, 2015 by Oleg Nesterov Committed by Linus Torvalds Apr 17, 2015

ptrace: fix race between ptrace_resume() and wait_task_stopped()



ptrace_resume() is called when the tracee is still __TASK_TRACED.  We set
tracee->exit_code and then wake_up_state() changes tracee->state.  If the
tracer's sub-thread does wait() in between, task_stopped_code(ptrace => T)
wrongly looks like another report from tracee.

This confuses debugger, and since wait_task_stopped() clears ->exit_code
the tracee can miss a signal.

Test-case:

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

	int pid;

	void *waiter(void *arg)
	{
		int stat;

		for (;;) {
			assert(pid == wait(&stat));
			assert(WIFSTOPPED(stat));
			if (WSTOPSIG(stat) == SIGHUP)
				continue;

			assert(WSTOPSIG(stat) == SIGCONT);
			printf("ERR! extra/wrong report:%x\n", stat);
		}
	}

	int main(void)
	{
		pthread_t thread;

		pid = fork();
		if (!pid) {
			assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
			for (;;)
				kill(getpid(), SIGHUP);
		}

		assert(pthread_create(&thread, NULL, waiter, NULL) == 0);

		for (;;)
			ptrace(PTRACE_CONT, pid, 0, SIGCONT);

		return 0;
	}

Note for stable: the bug is very old, but without 9899d11f "ptrace:
ensure arch_ptrace/ptrace_request can never race with SIGKILL" the fix
should use lock_task_sighand(child).

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Pavel Labath <labath@google.com>
Tested-by: Pavel Labath <labath@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 8de560de

kernel/ptrace.c

+20 −0

Original line number	Diff line number	Diff line
		@@ -697,6 +697,8 @@ static int ptrace_peek_siginfo(struct task_struct *child,
		static int ptrace_resume(struct task_struct *child, long request,
		unsigned long data)
		{
		bool need_siglock;

		if (!valid_signal(data))
		return -EIO;

		@@ -724,8 +726,26 @@ static int ptrace_resume(struct task_struct *child, long request,
		user_disable_single_step(child);
		}

		/*
		* Change ->exit_code and ->state under siglock to avoid the race
		* with wait_task_stopped() in between; a non-zero ->exit_code will
		* wrongly look like another report from tracee.
		*
		* Note that we need siglock even if ->exit_code == data and/or this
		* status was not reported yet, the new status must not be cleared by
		* wait_task_stopped() after resume.
		*
		* If data == 0 we do not care if wait_task_stopped() reports the old
		* status and clears the code too; this can't race with the tracee, it
		* takes siglock after resume.
		*/
		need_siglock = data && !thread_group_empty(current);
		if (need_siglock)
		spin_lock_irq(&child->sighand->siglock);
		child->exit_code = data;
		wake_up_state(child, __TASK_TRACED);
		if (need_siglock)
		spin_unlock_irq(&child->sighand->siglock);

		return 0;
		}