[PATCH] hrtimers: prevent possible itimer DoS

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>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 47 insertions, 25 deletions

diff --git a/kernel/itimer.c b/kernel/itimer.c
index 4fc6c0caf5d4..307c6a632ef6 100644
--- a/kernel/itimer.c
+++ b/kernel/itimer.c
@@ -135,11 +135,6 @@ enum hrtimer_restart it_real_fn(struct hrtimer *timer)
 
         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:
diff --git a/kernel/signal.c b/kernel/signal.c
index 8072e568bbe0..e2a7d4bf7d57 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -456,26 +456,50 @@ static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
 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){