[RFC PATCH] ntp: Avoid undefined behaviour in second_overflow()

[Xiongfeng Wang]

When I ran Syzkaller testsuite, I got the following call trace.
================================================================================
UBSAN: Undefined behaviour in kernel/time/ntp.c:457:16
signed integer overflow:
9223372036854775807 + 500 cannot be represented in type 'long int'
CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.19.25-dirty #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xca/0x13e lib/dump_stack.c:113
 ubsan_epilogue+0xe/0x81 lib/ubsan.c:159
 handle_overflow+0x193/0x1e2 lib/ubsan.c:190
 second_overflow+0x403/0x540 kernel/time/ntp.c:457
 accumulate_nsecs_to_secs kernel/time/timekeeping.c:2002 [inline]
 logarithmic_accumulation kernel/time/timekeeping.c:2046 [inline]
 timekeeping_advance+0x2bb/0xec0 kernel/time/timekeeping.c:2114
 tick_do_update_jiffies64.part.2+0x1a0/0x350 kernel/time/tick-sched.c:97
 tick_do_update_jiffies64 kernel/time/tick-sched.c:1229 [inline]
 tick_nohz_update_jiffies kernel/time/tick-sched.c:499 [inline]
 tick_nohz_irq_enter kernel/time/tick-sched.c:1232 [inline]
 tick_irq_enter+0x1fd/0x240 kernel/time/tick-sched.c:1249
 irq_enter+0xc4/0x100 kernel/softirq.c:353
 entering_irq arch/x86/include/asm/apic.h:517 [inline]
 entering_ack_irq arch/x86/include/asm/apic.h:523 [inline]
 smp_apic_timer_interrupt+0x20/0x480 arch/x86/kernel/apic/apic.c:1052
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:864
 </IRQ>
RIP: 0010:native_safe_halt+0x2/0x10 arch/x86/include/asm/irqflags.h:58
Code: 01 f0 0f 82 bc fd ff ff 48 c7 c7 c0 21 b1 83 e8 a1 0a 02 ff e9 ab fd ff ff 4c 89 e7 e8 77 b6 a5 fe e9 6a ff ff ff 90 90 fb f4 <c3> 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 f4 c3 90 90 90 90 90 90
RSP: 0018:ffff888106307d20 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
RAX: 0000000000000007 RBX: dffffc0000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8881062e4f1c
RBP: 0000000000000003 R08: ffffed107c5dc77b R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff848c78a0
R13: 0000000000000003 R14: 1ffff11020c60fae R15: 0000000000000000
 arch_safe_halt arch/x86/include/asm/paravirt.h:94 [inline]
 default_idle+0x24/0x2b0 arch/x86/kernel/process.c:561
 cpuidle_idle_call kernel/sched/idle.c:153 [inline]
 do_idle+0x2ca/0x420 kernel/sched/idle.c:262
 cpu_startup_entry+0xcb/0xe0 kernel/sched/idle.c:368
 start_secondary+0x421/0x570 arch/x86/kernel/smpboot.c:271
 secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:243
================================================================================

It is because time_maxerror is set as 0x7FFFFFFFFFFFFFFF by user. It
overflows when we add it with 'MAXFREQ / NSEC_PER_USEC' in
'second_overflow()'.

This patch add a limit check and saturate it when the user set
'time_maxerror'.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
---
 kernel/time/ntp.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 36a2bef..38e1b65 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -677,6 +677,8 @@ static inline void process_adjtimex_modes(const struct timex *txc, s32 *time_tai
 
 	if (txc->modes & ADJ_MAXERROR)
 		time_maxerror = txc->maxerror;
+	if (time_maxerror > NTP_PHASE_LIMIT)
+		time_maxerror = NTP_PHASE_LIMIT;
 
 	if (txc->modes & ADJ_ESTERROR)
 		time_esterror = txc->esterror;
-- 
1.7.12.4