CVE-2018-12896 (published 2018-07-02T17:29Z)

An issue was discovered in the Linux kernel through 4.17.3. An Integer Overflow in kernel/time/posix-timers.c in the POSIX timer code is caused by the way the overrun accounting works. Depending on interval and expiry time values, the overrun can be larger than INT_MAX, but the accounting is int based. This basically makes the accounting values, which are visible to user space via timer_getoverrun(2) and siginfo::si_overrun, random. For example, a local user can cause a denial of service (signed integer overflow) via crafted mmap, futex, timer_create, and timer_settime system calls.

Severity: LOW
CVSS v2 base score: 2.1 (AV:L/AC:L/Au:N/C:N/I:N/A:P)
CVSS v3 base score: 5.5, MEDIUM (CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: CWE-190
cwe_name: Integer Overflow or Wraparound
description: The product performs a calculation that can
         produce an integer overflow or wraparound when the logic
         assumes that the resulting value will always be larger than
         the original value. This occurs when an integer value is
         incremented to a value that is too large to store in the
         associated representation. When this occurs, the value may
         become a very small or negative number.
extended_description: 
            
            
            
            
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Thomas Gleixner
author_date: 2018-06-26 15:21:32+02:00
msg: posix-timers: Sanitize overrun handling

The posix timer overrun handling is broken because the forwarding functions
can return a huge number of overruns which does not fit in an int. As a
consequence timer_getoverrun(2) and siginfo::si_overrun can turn into
random number generators.

The k_clock::timer_forward() callbacks return a 64 bit value now. Make
k_itimer::ti_overrun[_last] 64bit as well, so the kernel internal
accounting is correct. 3Remove the temporary (int) casts.

Add a helper function which clamps the overrun value returned to user space
via timer_getoverrun(2) or siginfo::si_overrun limited to a positive value
between 0 and INT_MAX. INT_MAX is an indicator for user space that the
overrun value has been clamped.

Reported-by: Team OWL337 <icytxw@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Link: https://lkml.kernel.org/r/20180626132705.018623573@linutronix.de
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: posix-timers.h
change_type: ModificationType.MODIFY
diff: @@ -95,8 +95,8 @@ struct k_itimer {
 	clockid_t		it_clock;
 	timer_t			it_id;
 	int			it_active;
-	int			it_overrun;
-	int			it_overrun_last;
+	s64			it_overrun;
+	s64			it_overrun_last;
 	int			it_requeue_pending;
 	int			it_sigev_notify;
 	ktime_t			it_interval;

programming_language: C
filename: posix-cpu-timers.c
change_type: ModificationType.MODIFY
diff: @@ -85,7 +85,7 @@ static void bump_cpu_timer(struct k_itimer *timer, u64 now)
 			continue;
 
 		timer->it.cpu.expires += incr;
-		timer->it_overrun += 1 << i;
+		timer->it_overrun += 1LL << i;
 		delta -= incr;
 	}
 }

programming_language: C
filename: posix-timers.c
change_type: ModificationType.MODIFY
diff: @@ -283,6 +283,17 @@ static __init int init_posix_timers(void)
 }
 __initcall(init_posix_timers);
 
+/*
+ * The siginfo si_overrun field and the return value of timer_getoverrun(2)
+ * are of type int. Clamp the overrun value to INT_MAX
+ */
+static inline int timer_overrun_to_int(struct k_itimer *timr, int baseval)
+{
+	s64 sum = timr->it_overrun_last + (s64)baseval;
+
+	return sum > (s64)INT_MAX ? INT_MAX : (int)sum;
+}
+
 static void common_hrtimer_rearm(struct k_itimer *timr)
 {
 	struct hrtimer *timer = &timr->it.real.timer;
@@ -290,9 +301,8 @@ static void common_hrtimer_rearm(struct k_itimer *timr)
 	if (!timr->it_interval)
 		return;
 
-	timr->it_overrun += (unsigned int) hrtimer_forward(timer,
-						timer->base->get_time(),
-						timr->it_interval);
+	timr->it_overrun += hrtimer_forward(timer, timer->base->get_time(),
+					    timr->it_interval);
 	hrtimer_restart(timer);
 }
 
@@ -321,10 +331,10 @@ void posixtimer_rearm(struct siginfo *info)
 
 		timr->it_active = 1;
 		timr->it_overrun_last = timr->it_overrun;
-		timr->it_overrun = -1;
+		timr->it_overrun = -1LL;
 		++timr->it_requeue_pending;
 
-		info->si_overrun += timr->it_overrun_last;
+		info->si_overrun = timer_overrun_to_int(timr, info->si_overrun);
 	}
 
 	unlock_timer(timr, flags);
@@ -418,9 +428,8 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer)
 					now = ktime_add(now, kj);
 			}
 #endif
-			timr->it_overrun += (unsigned int)
-				hrtimer_forward(timer, now,
-						timr->it_interval);
+			timr->it_overrun += hrtimer_forward(timer, now,
+							    timr->it_interval);
 			ret = HRTIMER_RESTART;
 			++timr->it_requeue_pending;
 			timr->it_active = 1;
@@ -524,7 +533,7 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
 	new_timer->it_id = (timer_t) new_timer_id;
 	new_timer->it_clock = which_clock;
 	new_timer->kclock = kc;
-	new_timer->it_overrun = -1;
+	new_timer->it_overrun = -1LL;
 
 	if (event) {
 		rcu_read_lock();
@@ -702,7 +711,7 @@ void common_timer_get(struct k_itimer *timr, struct itimerspec64 *cur_setting)
 	 * expiry time forward by intervals, so expiry is > now.
 	 */
 	if (iv && (timr->it_requeue_pending & REQUEUE_PENDING || sig_none))
-		timr->it_overrun += (int)kc->timer_forward(timr, now);
+		timr->it_overrun += kc->timer_forward(timr, now);
 
 	remaining = kc->timer_remaining(timr, now);
 	/* Return 0 only, when the timer is expired and not pending */
@@ -791,7 +800,7 @@ SYSCALL_DEFINE1(timer_getoverrun, timer_t, timer_id)
 	if (!timr)
 		return -EINVAL;
 
-	overrun = timr->it_overrun_last;
+	overrun = timer_overrun_to_int(timr, 0);
 	unlock_timer(timr, flags);
 
 	return overrun;

programming_language: C