linux-kernel - Re: [PATCH] posix-timers: fix stack info leak in timer

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Message-ID: <20141005210614.GA28899@redhat.com>
Date:	Sun, 5 Oct 2014 23:06:14 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mathias Krause <minipli@...glemail.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Brad Spengler <spender@...ecurity.net>,
	PaX Team <pageexec@...email.hu>
Subject: Re: [PATCH] posix-timers: fix stack info leak in timer_create()

On 10/04, Mathias Krause wrote:
>
> If userland creates a timer without specifying a sigevent info, we'll
> create one ourself, using a stack local variable. Particularly will we
> use the timer ID as sival_int. But as sigev_value is a union containing
> a pointer and an int, that assignment will only partially initialize
> sigev_value on systems where the size of a pointer is bigger than the
> size of an int. On such systems we'll copy the uninitialized stack bytes
> from the timer_create() call to userland when the timer actually fires
> and we're going to deliver the signal.

So we have a minor information leak,

> Cc: <stable@...r.kernel.org>	# v2.6.28+

not sure this is -stable material but I won't really argue.

> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -636,6 +636,7 @@ SYSCALL_DEFINE3(timer_create, const clockid_t, which_clock,
>  			goto out;
>  		}
>  	} else {
> +		memset(&event.sigev_value, 0, sizeof(event.sigev_value));
>  		event.sigev_notify = SIGEV_SIGNAL;
>  		event.sigev_signo = SIGALRM;
>  		event.sigev_value.sival_int = new_timer->it_id;

How about

	-	event.sigev_value.sival_int = new_timer->it_id;
	+	event.sigev_value = (sigval_t) { .sival_int = new_timer_id };

?

(btw, new_timer->sigq->info.si_tid initialization can use new_timer_id too)

this makes the initialization more explicit and can help gcc to optimize
this assignment although this is minor.

In any case this all looks confusing to me. sys_timer_create() does

	new_timer->sigq->info.si_value = event.sigev_value;
	new_timer->sigq->info.si_tid   = new_timer->it_id;

later, this writes to the differents members (_rt and _timer) in the
same union. But the comment in struct siginfo says that we should use
_timer. And copy_siginfo_to_user() reports si_tid and si_ptr, this
again reads _timer and _rt. This should actually work, _sigval should
have the same offset in both struct's, still it looks confusing imho.
Perhaps we should change

	#define si_value	_sifields._rt._sigval
	#define si_int		_sifields._rt._sigval.sival_int
	#define si_ptr		_sifields._rt._sigval.sival_ptr

to use _timer instead. Nevermind, this is off-topic.

Oleg.

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