linux-kernel - Re: [PATCH] pstore: use ktime_get_real_fast_ns() instead of _

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Message-ID: <CAGXu5jKs2UWcjLwMq+_94etrQhiZs=x79tL251d1L=5t4DYkUg@mail.gmail.com>
Date:   Thu, 9 Nov 2017 17:04:01 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Arnd Bergmann <arnd@...db.de>, Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pstore: use ktime_get_real_fast_ns() instead of __getnstimeofday()

On Thu, Nov 9, 2017 at 4:46 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 10 Nov 2017, Arnd Bergmann wrote:
>> On Fri, Nov 10, 2017 at 12:00 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> > Hmm, no. None of the regular accessor functions can be called from NMI
>> > context safely.
>>
>> Right, that's what I mean: it must not get called from NMI context, but it
>> currently is, at least for this case:
>>
>> NMI handler:
>>   something bad
>>     panic()
>>       kmsg_dump()
>>         pstore_dump()
>>            pstore_record_init()
>>              __getnstimeofday()
>>
>> I should probably add that to the changelog text ;-)
>
> Indeed.

Er, so, is this safe to call there? I've had to fix this a few times
now, so if using ktime_get_real_fast_ns() can be used here (and
doesn't return 0) then this is easily an improvement over the existing
"maybe read 0" case pstore has now.

-Kees


-- 
Kees Cook
Pixel Security