linux-kernel - Re: time: signed integer overflow in ktime_add

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Date:	Fri, 4 Dec 2015 14:44:55 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: time: signed integer overflow in ktime_add_safe



On 12/04/2015 02:33 PM, Dmitry Vyukov wrote:
> On Fri, Dec 4, 2015 at 12:32 PM, Andrey Ryabinin <ryabinin.a.a@...il.com> wrote:
>> On 12/04/2015 02:05 PM, Dmitry Vyukov wrote:
>>> Hello,
>>>
>>> UBSAN reports undefined behavior in ktime_add_safe:
>>>
>>> UBSAN: Undefined behaviour in kernel/time/hrtimer.c:310:16
>>> signed integer overflow:
>>> 9223372036854775807 + 100000000 cannot be represented in type 'long long int'
>>> CPU: 3 PID: 26438 Comm: syzkaller_execu Tainted: G    B
>>> 4.4.0-rc3+ #141
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>>>  0000000000000003 ffff88005a62f518 ffffffff82c65588 0000000041b58ab3
>>>  ffffffff8769c1b6 ffffffff82c654d6 ffff88005a62f4e0 ffff88005a62f618
>>>  0000000005f5e100 0000000000000001 ffff88005a62f520 ffffffff82d540c7
>>> Call Trace:
>>>  [<ffffffff82d54f69>] __ubsan_handle_add_overflow+0x2a/0x31 lib/ubsan.c:199
>>>  [<     inline     >] ktime_add_safe kernel/time/hrtimer.c:310
>>>  [<     inline     >] hrtimer_set_expires_range_ns include/linux/hrtimer.h:224
>>>  [<ffffffff86820fce>] schedule_hrtimeout_range_clock+0x4ae/0x580
>>> kernel/time/hrtimer.c:1731
>>>  [<ffffffff868210ca>] schedule_hrtimeout_range+0x2a/0x40
>>> kernel/time/hrtimer.c:1779
>>>  [<ffffffff81833112>] poll_schedule_timeout+0xd2/0x180 fs/select.c:241
>>>  [<     inline     >] do_poll fs/select.c:861
>>>  [<ffffffff8183706b>] do_sys_poll+0xa4b/0xfc0 fs/select.c:911
>>>  [<     inline     >] SYSC_ppoll fs/select.c:1019
>>>  [<ffffffff81837d79>] SyS_ppoll+0x1a9/0x420 fs/select.c:991
>>>
>>> On commit 31ade3b83e1821da5fbb2f11b5b3d4ab2ec39db8.
>>>
>>> For:
>>>
>>> ktime_t ktime_add_safe(const ktime_t lhs, const ktime_t rhs)
>>> {
>>>         ktime_t res = ktime_add(lhs, rhs);
>>>         if (res.tv64 < 0 || res.tv64 < lhs.tv64 || res.tv64 < rhs.tv64)
>>>                 res = ktime_set(KTIME_SEC_MAX, 0);
>>>         return res;
>>> }
>>>
>>
>> I think we can workaround it this way:
>>
>> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
>> index 2b6a204..c768cc0 100644
>> --- a/include/linux/ktime.h
>> +++ b/include/linux/ktime.h
>> @@ -61,7 +61,7 @@ static inline ktime_t ktime_set(const s64 secs, const unsigned long nsecs)
>>
>>  /* Add two ktime_t variables. res = lhs + rhs: */
>>  #define ktime_add(lhs, rhs) \
>> -               ({ (ktime_t){ .tv64 = (lhs).tv64 + (rhs).tv64 }; })
>> +               ({ (ktime_t){ .tv64 = (s64)((u64)(lhs).tv64 + (u64)(rhs).tv64) }; })
>>
>>  /*
>>   * Add a ktime_t variable and a scalar nanosecond value.
>>
>>> compiler is within its rights to assume that res.tv64 < rhs.tv64 is
>>> always false (after inlining ktime_add). And compilers already do
>>> this.
>>
>> Not with -fno-strict-overflow
> 
> 
> Then I guess we need to disable this check in kernel ubsan.
> 

I'm not so sure. It finds real bugs, e.g. 32a8df4e0b33f ("sched: Fix odd values in effective_load() calculations")
was caught by UBSAN
I guess that we could fix most signed overflows simply by casting to unsigned type.

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