linux-kernel - Re: [PATCH 8/8] selftests/proc: Assert clock_gettime(CLOCK

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Date:   Sun, 26 Mar 2023 22:03:56 +0200
From:   Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Wei Li <liwei391@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Yu Liao <liaoyu15@...wei.com>, Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 8/8] selftests/proc: Assert clock_gettime(CLOCK_BOOTTIME)
 VS /proc/uptime monotonicity

On 21. 03. 2023. 13:44, Frederic Weisbecker wrote:
> On Wed, Mar 08, 2023 at 04:59:41PM +0100, Mirsad Todorovac wrote:
>> On 2/22/23 15:46, Frederic Weisbecker wrote:
>> From what I see, you round the CLOCK_BOOTIME time to 1/100ths of a second.
>>
>> A simple program that queries clock_getres() on system clocks gives this
>> result:
>>
>> clock_res [CLOCK_REALTIME] = 0.000000001s
>> clock_res [CLOCK_REALTIME_COARSE] = 0.004000000s
>> clock_res [CLOCK_MONOTONIC] = 0.000000001s
>> clock_res [CLOCK_MONOTONIC_COARSE] = 0.004000000s
>> clock_res [CLOCK_MONOTONIC_RAW] = 0.000000001s
>> clock_res [CLOCK_BOOTTIME] = 0.000000001s
>> clock_res [CLOCK_PROCESS_CPUTIME_ID] = 0.000000001s
>> clock_res [CLOCK_THREAD_CPUTIME_ID] = 0.000000001s
>>
>> A number of programs may depend i.e. on CLOCK_REALTIME or CLOCK_BOOTIME to give
>> different result each nanosecond.
>>
>> I came across this when generating nonces for HMACs according to recommendations
>> from RFC 4086 "Randomness Requirements for Security".
>>
>> If the value of CLOCK_BOOTTIME or CLOCK_REALTIME is incremented not in what
>> clock_getres() gives, but at best in 1/100th of second instead, that would seriously
>> weaken our security (for as you know, in many cryptographic uses nonces need not
>> be random, but MUST NOT ever repeat nor go backwards).
>>
>> Could we modify the test for this assumption, or is the assumption wrong?
>>
>> Here the test for CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID
>> increasing monotonically with guaranteed increased value of nanoseconds
>> would also seem good.
>>
>> Maybe this is already covered in another test case, but it seems that all
>> clocks should be guaranteed to be monotonically increasing, and increased
>> at least by one nanosecond with each syscall, or many algorithms would break.
>>
>> In other words, CLOCK_BOOTTIME should be tested to increase monotonically in
>> the resolution given by clock_getres (CLOCK_BOOTTIME, &tv_res), not in 1/100ths
>> of second (IMHO).
> 
> Maybe but verifying a clock against its own resolution is another testcase. Here the
> point is to verify that CLOCK_BOOTTIME is monotonic against /proc/uptime, and
> since /proc/uptime has an 1/100 second resolution, rounding clock_gettime(CLOCK_BOOTTIME)
> result down to that is the best we can do.
> 
> Thanks.

Hi Frederic,

Thank you for explaining that.

I've read somewhere (forgot the link) that clock_gettime(CLOCK_*) clocks should
be guaranteed to return at least a nanosecond increased value for a PID or TID
from call to call.

Returning the same value would break some algorithms that depend on monotonous
increase of time - for example, some naive implementations of nonce generators.

I believe this is worth assuring in tests, or possibly some naive crypto
would reveal its pre-shared secrets in consecutive calls (Please see RFC 4086,
"Randomness Requirements for Security" for greater detail in explanation.

Best regards,
Mirsad

-- 
Mirsad Goran Todorovac
Sistem inženjer
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu
 
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb, Republic of Croatia
The European Union