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Subject[patch 43/46] hrtimers: prevent possible itimer DoS
From: Thomas Gleixner <tglx@linutronix.de>

Fix potential setitimer DoS with high-res timers by pushing itimer rearm
processing to process context.

[Fixes from: Ingo Molnar <mingo@elte.hu>]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

kernel/itimer.c | 14 +++++--------
kernel/signal.c | 58 +++++++++++++++++++++++++++++++++++++++-----------------
2 files changed, 47 insertions(+), 25 deletions(-)

Index: linux-2.6.20-rc4-mm1-bo/kernel/itimer.c
===================================================================
--- linux-2.6.20-rc4-mm1-bo.orig/kernel/itimer.c
+++ linux-2.6.20-rc4-mm1-bo/kernel/itimer.c
@@ -135,11 +135,6 @@ enum hrtimer_restart it_real_fn(struct h

send_group_sig_info(SIGALRM, SEND_SIG_PRIV, sig->tsk);

- if (sig->it_real_incr.tv64 != 0) {
- hrtimer_forward(timer, hrtimer_cb_get_time(timer),
- sig->it_real_incr);
- return HRTIMER_RESTART;
- }
return HRTIMER_NORESTART;
}

@@ -231,11 +226,14 @@ again:
spin_unlock_irq(&tsk->sighand->siglock);
goto again;
}
- tsk->signal->it_real_incr =
- timeval_to_ktime(value->it_interval);
expires = timeval_to_ktime(value->it_value);
- if (expires.tv64 != 0)
+ if (expires.tv64 != 0) {
+ tsk->signal->it_real_incr =
+ timeval_to_ktime(value->it_interval);
hrtimer_start(timer, expires, HRTIMER_MODE_REL);
+ } else
+ tsk->signal->it_real_incr.tv64 = 0;
+
spin_unlock_irq(&tsk->sighand->siglock);
break;
case ITIMER_VIRTUAL:
Index: linux-2.6.20-rc4-mm1-bo/kernel/signal.c
===================================================================
--- linux-2.6.20-rc4-mm1-bo.orig/kernel/signal.c
+++ linux-2.6.20-rc4-mm1-bo/kernel/signal.c
@@ -456,26 +456,50 @@ static int __dequeue_signal(struct sigpe
int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
{
int signr = __dequeue_signal(&tsk->pending, mask, info);
- if (!signr)
+ if (!signr) {
signr = __dequeue_signal(&tsk->signal->shared_pending,
mask, info);
+ /*
+ * itimer signal ?
+ *
+ * itimers are process shared and we restart periodic
+ * itimers in the signal delivery path to prevent DoS
+ * attacks in the high resolution timer case. This is
+ * compliant with the old way of self restarting
+ * itimers, as the SIGALRM is a legacy signal and only
+ * queued once. Changing the restart behaviour to
+ * restart the timer in the signal dequeue path is
+ * reducing the timer noise on heavy loaded !highres
+ * systems too.
+ */
+ if (unlikely(signr == SIGALRM)) {
+ struct hrtimer *tmr = &tsk->signal->real_timer;
+
+ if (!hrtimer_is_queued(tmr) &&
+ tsk->signal->it_real_incr.tv64 != 0) {
+ hrtimer_forward(tmr, tmr->base->get_time(),
+ tsk->signal->it_real_incr);
+ hrtimer_restart(tmr);
+ }
+ }
+ }
recalc_sigpending_tsk(tsk);
- if (signr && unlikely(sig_kernel_stop(signr))) {
- /*
- * Set a marker that we have dequeued a stop signal. Our
- * caller might release the siglock and then the pending
- * stop signal it is about to process is no longer in the
- * pending bitmasks, but must still be cleared by a SIGCONT
- * (and overruled by a SIGKILL). So those cases clear this
- * shared flag after we've set it. Note that this flag may
- * remain set after the signal we return is ignored or
- * handled. That doesn't matter because its only purpose
- * is to alert stop-signal processing code when another
- * processor has come along and cleared the flag.
- */
- if (!(tsk->signal->flags & SIGNAL_GROUP_EXIT))
- tsk->signal->flags |= SIGNAL_STOP_DEQUEUED;
- }
+ if (signr && unlikely(sig_kernel_stop(signr))) {
+ /*
+ * Set a marker that we have dequeued a stop signal. Our
+ * caller might release the siglock and then the pending
+ * stop signal it is about to process is no longer in the
+ * pending bitmasks, but must still be cleared by a SIGCONT
+ * (and overruled by a SIGKILL). So those cases clear this
+ * shared flag after we've set it. Note that this flag may
+ * remain set after the signal we return is ignored or
+ * handled. That doesn't matter because its only purpose
+ * is to alert stop-signal processing code when another
+ * processor has come along and cleared the flag.
+ */
+ if (!(tsk->signal->flags & SIGNAL_GROUP_EXIT))
+ tsk->signal->flags |= SIGNAL_STOP_DEQUEUED;
+ }
if ( signr &&
((info->si_code & __SI_MASK) == __SI_TIMER) &&
info->si_sys_private){
--

-
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