Code: COMPAT_SYSCALL_DEFINE5(waitid,
		int, which, compat_pid_t, pid,
		struct compat_siginfo __user *, infop, int, options,
		struct compat_rusage __user *, uru)
{
	struct rusage ru;
	struct waitid_info info = {.status = 0};
	long err = kernel_waitid(which, pid, &info, options, uru ? &ru : NULL);
	int signo = 0;
 	if (err > 0) {
 		signo = SIGCHLD;
 		err = 0;
	}
	if (!err && uru) {
		/* kernel_waitid() overwrites everything in ru */
		if (COMPAT_USE_64BIT_TIME)
			err = copy_to_user(uru, &ru, sizeof(ru));
		else
			err = put_compat_rusage(&ru, uru);
		if (err)
			return -EFAULT;
 	}
 
 	if (!infop)
		return err;

	user_access_begin();
	unsafe_put_user(signo, &infop->si_signo, Efault);
	unsafe_put_user(0, &infop->si_errno, Efault);
	unsafe_put_user(info.cause, &infop->si_code, Efault);
	unsafe_put_user(info.pid, &infop->si_pid, Efault);
	unsafe_put_user(info.uid, &infop->si_uid, Efault);
	unsafe_put_user(info.status, &infop->si_status, Efault);
	user_access_end();
	return err;
Efault:
	user_access_end();
	return -EFAULT;
}

Commit Message: fix infoleak in waitid(2)

kernel_waitid() can return a PID, an error or 0.  rusage is filled in the first
case and waitid(2) rusage should've been copied out exactly in that case, *not*
whenever kernel_waitid() has not returned an error.  Compat variant shares that
braino; none of kernel_wait4() callers do, so the below ought to fix it.

Reported-and-tested-by: Alexander Potapenko <glider@google.com>
Fixes: ce72a16fa705 ("wait4(2)/waitid(2): separate copying rusage to userland")
Cc: stable@vger.kernel.org # v4.13
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
CWE ID: CWE-200
COMPAT_SYSCALL_DEFINE5(waitid,
		int, which, compat_pid_t, pid,
		struct compat_siginfo __user *, infop, int, options,
		struct compat_rusage __user *, uru)
{
	struct rusage ru;
	struct waitid_info info = {.status = 0};
	long err = kernel_waitid(which, pid, &info, options, uru ? &ru : NULL);
	int signo = 0;
 	if (err > 0) {
 		signo = SIGCHLD;
 		err = 0;
		if (uru) {
			/* kernel_waitid() overwrites everything in ru */
			if (COMPAT_USE_64BIT_TIME)
				err = copy_to_user(uru, &ru, sizeof(ru));
			else
				err = put_compat_rusage(&ru, uru);
			if (err)
				return -EFAULT;
		}
 	}
 
 	if (!infop)
		return err;

	user_access_begin();
	unsafe_put_user(signo, &infop->si_signo, Efault);
	unsafe_put_user(0, &infop->si_errno, Efault);
	unsafe_put_user(info.cause, &infop->si_code, Efault);
	unsafe_put_user(info.pid, &infop->si_pid, Efault);
	unsafe_put_user(info.uid, &infop->si_uid, Efault);
	unsafe_put_user(info.status, &infop->si_status, Efault);
	user_access_end();
	return err;
Efault:
	user_access_end();
	return -EFAULT;
}
