Re: Fix hibernation in FIPS mode?

On Thu, Apr 1, 2021 at 3:54 PM Ard Biesheuvel [off-list ref] wrote:

On Thu, 1 Apr 2021 at 15:38, Rafael J. Wysocki [off-list ref] wrote:

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On Thu, Apr 1, 2021 at 10:47 AM Ard Biesheuvel [off-list ref] wrote:

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On Tue, 30 Mar 2021 at 21:56, Simo Sorce [off-list ref] wrote:

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On Tue, 2021-03-30 at 21:45 +0200, Ard Biesheuvel wrote:

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On Tue, 30 Mar 2021 at 20:05, Simo Sorce [off-list ref] wrote:

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On Tue, 2021-03-30 at 16:46 +0200, Rafael J. Wysocki wrote:

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On Tue, Mar 30, 2021 at 12:14 AM Dexuan Cui [off-list ref] wrote:

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Hi,
MD5 was marked incompliant with FIPS in 2009:
a3bef3a31a19 ("crypto: testmgr - Skip algs not flagged fips_allowed in fips mode")
a1915d51e8e7 ("crypto: testmgr - Mark algs allowed in fips mode")

But hibernation_e820_save() is still using MD5, and fails in FIPS mode
due to the 2018 patch:
749fa17093ff ("PM / hibernate: Check the success of generating md5 digest before hibernation")

As a result, hibernation doesn't work when FIPS is on.

Do you think if hibernation_e820_save() should be changed to use a
FIPS-compliant algorithm like SHA-1?

I would say yes, it should.

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PS, currently it looks like FIPS mode is broken in the mainline:
https://www.mail-archive.com/linux-crypto@vger.kernel.org/msg49414.html

FYI, SHA-1 is not a good choice, it is only permitted in HMAC
constructions and only for specified uses. If you need to change
algorithm you should go straight to SHA-2 or SHA-3 based hashes.

What is the reason for using a [broken] cryptographic hash here? if
this is just an integrity check, better use CRC32

Not really.

CRC32 is not really sufficient for integrity checking here AFAICS.  It
might be made a fallback option if MD5 is not available, but making it
the default would be somewhat over the top IMO.

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If the integrity check is used exclusively to verify there were no
accidental changes and is not used as a security measure, by all means
I agree that using crc32 is a better idea.

Looking at 62a03defeabd58f74e07ca030d6c21e069d4d88e which introduced
this, it is only a best effort check which is simply omitted if md5
happens to be unavailable, so there is definitely no need for crypto
here.

Yes, it is about integrity checking only.  No, CRC32 is not equivalent
to MD5 in that respect AFAICS.

There are two possibilities:
- we care about an adversary attempting to forge a collision, in which
case you need a cryptographic hash which is not broken;
- we only care about integrity, in which case crypto is overkill, and
CRC32 is sufficient. (Note that the likelihood of an honest,
inadvertent modification not being caught by CRC32 is 1 in 4 billion)

That depends on how you count.

Surely, there are modifications caught by MD5 that will not be caught by CRC32.

MD5 does not meet either requirement, given that it is known to be
broken, and overkill for simple integrity checks. MD5 should be phased
out and removed, and moving this code onto the correct abstraction
would be a reasonable step towards that goal.

This clearly is a matter of opinion.

I'm not religious about it though.  If there is a general consensus
that CRC32 is sufficient for error detection in hibernation files,
then it can be used.  So is there such a consensus and if so, can you
give me a pointer to some research that it is based on?