linux-kernel - Re: UBSAN: Undefined behaviour in kernel/time/tick-sched.c:600:9

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Date:   Fri, 10 Mar 2017 22:14:56 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Meelis Roos <mroos@...ux.ee>,
        Linux Kernel list <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: UBSAN: Undefined behaviour in kernel/time/tick-sched.c:600:9

On Fri, Mar 10, 2017 at 9:45 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 10 Mar 2017, Meelis Roos wrote:
>
> Cc'ing KASAN folks
>
>> (Retry with fixed address ox 86 list)
>>
>> This HP Netserver LT6000R, quad P3 Xeon. Trying
>> 4.11.0-rc1-00088-gec3b93a, I got the following UBSAN warning that was
>> not there in 4.10:
>>
>> [   14.820437] ================================================================================
>> [   14.820534] UBSAN: Undefined behaviour in kernel/time/tick-sched.c:600:9
>> [   14.820594] signed integer overflow:
>> [   14.820645] 11567464533 + 3985952 cannot be represented in type 'long long int'
>
> This does not make any sense:
>
>      11567464533 + 3985952 = 0x2b1798055 + 0x3cd220 = 0x2b1b65275
>
> which definitely fits into a 'long long int' aka 64 bit


Is there a reproducer? Or can you print the values right in
tick-sched.c with pr_err?

The code in ubsan.c looks more complex than I would expect from that
code. There is probably some potential for truncation:

static s_max get_signed_val(struct type_descriptor *type, unsigned long val)
{
        if (is_inline_int(type)) {
                unsigned extra_bits = sizeof(s_max)*8 - type_bit_width(type);
                return ((s_max)val) << extra_bits >> extra_bits;
        }
        if (type_bit_width(type) == 64)
                return *(s64 *)val;
        return *(s_max *)val;
}



>> [   14.820722] CPU: 3 PID: 135 Comm: startpar Not tainted 4.11.0-rc1-00088-gec3b93a #61
>> [   14.820793] Hardware name: Hewlett Packard HP NetServer/HP System Board, BIOS 4.06.46 PW 06/25/2003
>> [   14.820863] Call Trace:
>> [   14.820950]  ? dump_stack+0x45/0x6e
>> [   14.821022]  ? ubsan_epilogue+0xb/0x40
>> [   14.821081]  ? handle_overflow+0xb6/0xe0
>> [   14.821146]  ? smp_trace_apic_timer_interrupt+0x85/0x100
>> [   14.821208]  ? __ubsan_handle_add_overflow+0xf/0x20
>> [   14.821281]  ? get_cpu_idle_time_us+0x204/0x2b0
>> [   14.821350]  ? get_idle_time+0x57/0xb0
>> [   14.821405]  ? show_stat+0x217/0x10c0
>> [   14.821463]  ? seq_open+0x33/0xb0
>> [   14.821519]  ? __d_alloc+0x149/0x2c0
>> [   14.821574]  ? get_iowait_time+0xb0/0xb0
>> [   14.821628]  ? single_open+0x4d/0xb0
>> [   14.821685]  ? single_open_size+0x37/0xc0
>> [   14.821748]  ? path_openat+0x7d5/0x1bd0
>> [   14.821804]  ? seq_read+0xca/0x6f0
>> [   14.821872]  ? _copy_to_user+0x5e/0x90
>> [   14.821937]  ? cp_new_stat64+0x108/0x130
>> [   14.821991]  ? __seq_open_private+0x80/0x80
>> [   14.822062]  ? proc_reg_read+0x7c/0x120
>> [   14.822117]  ? proc_reg_write+0x120/0x120
>> [   14.822172]  ? __vfs_read+0x3e/0x240
>> [   14.822228]  ? vfs_read+0x92/0x1b0
>> [   14.822282]  ? SyS_read+0x45/0xa0
>> [   14.822338]  ? do_fast_syscall_32+0xb9/0x220
>> [   14.822395]  ? entry_SYSENTER_32+0x47/0x71
>> [   14.822447]
>> ================================================================================