linux-kernel - Re: [PATCH][v8] PM / hibernate: Verify the consistent of e820 memory map by md5 value

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Date:   Mon, 29 Aug 2016 15:41:34 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Chen Yu <yu.c.chen@...el.com>, Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lee@...gul.tnic, Chun-Yi <jlee@...e.com>
Subject: Re: [PATCH][v8] PM / hibernate: Verify the consistent of e820 memory
 map by md5 value

On Mon, Aug 29, 2016 at 6:59 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Aug 29, 2016 at 12:35:40AM +0800, Chen Yu wrote:
>> On some platforms, there is occasional panic triggered when trying to
>> resume from hibernation, a typical panic looks like:
>>
>> "BUG: unable to handle kernel paging request at ffff880085894000
>> IP: [<ffffffff810c5dc2>] load_image_lzo+0x8c2/0xe70"
>>
>> This is because e820 map has been changed by BIOS across
>> hibernation, and one of the page frames from first kernel
>> is right located in second kernel's unmapped region, so panic
>> comes out when accessing unmapped kernel address.
>>
>> In order to expose this issue earlier, the md5 hash of e820 map
>> is passed from suspend kernel to resume kernel, and the system will
>> trigger panic once it finds the md5 value of previous kernel is not
>> the same as current resume kernel.
>
> ... so basically now even the cases where it managed to resume would
> panic because the digests differ, even if the original panic condition
> doesn't trigger the bug, i.e. your Note 1 below.
>
> The more important question IMHO would be, can we resume our system
> successfully *even* if BIOS fiddled with the e820 map?
>
> We'd still warn the hell out of it and even make that the md5 digest
> comparison a default-enabled thing without even having a config option
> to disable it but can we try harder not to panic and deal with this next
> BIOS f*ckup more intelligently than throwing our hands in the air and
> giving up?

We need not panic in principle and I wouldn't do that.

I would warn and try to continue regardless (which was the original
plan here AFAICS), or we change a possible data loss into a guaranteed
one.

IMO it is sufficient to give up when a PFN we have image data for is
not pfn_valid() during resume, which we do already.

Thanks,
Rafael