linux-kernel - Re: [PATCH v6 1/1] use crc32 instead of md5 for hibernation e820 integrity check

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Date:   Mon, 12 Apr 2021 15:24:45 -0400
From:   Chris von Recklinghausen <crecklin@...hat.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     ardb@...nel.org, simo@...hat.com, rafael@...nel.org,
        decui@...rosoft.com, linux-pm@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/1] use crc32 instead of md5 for hibernation e820
 integrity check

On 4/12/21 3:20 PM, Eric Biggers wrote:
> On Mon, Apr 12, 2021 at 03:04:58PM -0400, Chris von Recklinghausen wrote:
>> On 4/12/21 1:45 PM, Eric Biggers wrote:
>>> On Mon, Apr 12, 2021 at 10:09:32AM -0400, Chris von Recklinghausen wrote:
>>>> Suspend fails on a system in fips mode because md5 is used for the e820
>>>> integrity check and is not available. Use crc32 instead.
>>>>
>>>> This patch changes the integrity check algorithm from md5 to crc32.
>>>>
>>>> The purpose of the integrity check is to detect possible differences
>>>> between the memory map used at the time when the hibernation image is
>>>> about to be loaded into memory and the memory map used at the image
>>>> creation time, because it is generally unsafe to load the image if the
>>>> current memory map doesn't match the one used when it was created. so
>>>> it is not intended as a cryptographic integrity check.
>>> This still doesn't actually explain why a non-cryptographic checksum is
>>> sufficient.  "Detection of possible differences" could very well require
>>> cryptographic authentication; it depends on whether malicious changes need to be
>>> detected or not.
>> Hi Eric,
>>
>> The cases that the commit comments for 62a03defeabd mention are the same as
>> for this patch, e.g.
>>
>>      1. Without this patch applied, it is possible that BIOS has
>>         provided an inconsistent memory map, but the resume kernel is still
>>         able to restore the image anyway(e.g, E820_RAM region is the superset
>>         of the previous one), although the system might be unstable. So this
>>         patch tries to treat any inconsistent e820 as illegal.
>>
>>      2. Another case is, this patch replies on comparing the e820_saved, but
>>         currently the e820_save might not be strictly the same across
>>         hibernation, even if BIOS has provided consistent e820 map - In
>>         theory mptable might modify the BIOS-provided e820_saved dynamically
>>         in early_reserve_e820_mpc_new, which would allocate a buffer from
>>         E820_RAM, and marks it from E820_RAM to E820_RESERVED).
>>         This is a potential and rare case we need to deal with in OS in
>>         the future.
>>
>> Maybe they should be added to the comments with this patch as well? In any
>> case, the above comments only mention detecting consequences of BIOS
>> issues/actions on the e820 map and not intrusions from attackers requiring
>> cryptographic protection. Does that seem to be a reasonable explanation to
>> you? If so I can add these to the commit comments.
>>
>> I'll make the other changes you suggest below.
>>
>> Thanks,
>>
> Those details are still missing the high-level point.  Is this just meant to
> detect non-malicious changes (presumably caused by BIOS bugs), or is it meant to
> detect malicious changes?  That's all that really needs to be mentioned.


Ok, I'll say the intent is to detect non-malicious changes presumably 
from BIOS issues.

Thanks,

Chris

>
> - Eric
>