Re: [PATCH] hrtimer: prevent overrun DoS in hrtimer_forward()

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 14 Mar 2007 11:00:12 +0100 Thomas Gleixner <tglx@linutronix.de> wrote:

> rtimer_forward() does not check for the possible overflow of
> timer->expires. This can happen on 64 bit machines with large interval
> values and results currently in an endless loop in the softirq because
> the expiry value becomes negative and therefor the timer is expired all
> the time.
> 
> Check for this condition and set the expiry value to the max. expiry
> time in the future.
> 
> The fix should be applied to stable kernel series as well.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix,de>
> 
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index ec4cb9f..5e7122d 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -644,6 +644,12 @@ hrtimer_forward(struct hrtimer *timer, k
>  		orun++;
>  	}
>  	timer->expires = ktime_add(timer->expires, interval);
> +	/*
> +	 * Make sure, that the result did not wrap with a very large
> +	 * interval.
> +	 */
> +	if (timer->expires.tv64 < 0)
> +		timer->expires = ktime_set(KTIME_SEC_MAX, 0);
>  
>  	return orun;
>  }

kernel/hrtimer.c: In function 'hrtimer_forward':
kernel/hrtimer.c:652: warning: overflow in implicit constant conversion

problem is, KTIME_SEC_MAX is 9,223,372,036 and ktime_set() takes a `long'.


This?

--- a/include/linux/ktime.h~ktime_set-fix-arg-type
+++ a/include/linux/ktime.h
@@ -72,13 +72,13 @@ typedef union {
  *
  * Return the ktime_t representation of the value
  */
-static inline ktime_t ktime_set(const long secs, const unsigned long nsecs)
+static inline ktime_t ktime_set(const s64 secs, const unsigned long nsecs)
 {
 #if (BITS_PER_LONG == 64)
 	if (unlikely(secs >= KTIME_SEC_MAX))
 		return (ktime_t){ .tv64 = KTIME_MAX };
 #endif
-	return (ktime_t) { .tv64 = (s64)secs * NSEC_PER_SEC + (s64)nsecs };
+	return (ktime_t) { .tv64 = secs * NSEC_PER_SEC + (s64)nsecs };
 }
 
 /* Subtract two ktime_t variables. rem = lhs -rhs: */
_

I worry about that `secs >= KTIME_SEC_MAX' comparison in there, too.  Both
operands are signed.