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Reply-To: crecklin@redhat.com
Subject: Re: [PATCH v5 1/1] use crc32 instead of md5 for hibernation e820
 integrity check
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     ardb@kernel.org, simo@redhat.com, rafael@kernel.org,
        decui@microsoft.com, linux-pm@vger.kernel.org,
        linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20210408131506.17941-1-crecklin@redhat.com>
 <YG8hgzBPQAvov9Vz@sol.localdomain>
From:   Chris von Recklinghausen <crecklin@redhat.com>
Organization: Red Hat
Message-ID: <f01021a0-2edd-da43-5f04-043b4f42be8b@redhat.com>
Date:   Thu, 8 Apr 2021 11:53:59 -0400
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On 4/8/21 11:30 AM, Eric Biggers wrote:
> On Thu, Apr 08, 2021 at 09:15:06AM -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. This integrity check is used only to verify accidental
>> corruption of the hybernation data and is not intended as a
>> cryptographic integrity check.
>> Md5 is overkill in this case and also disabled in FIPS mode because it
>> is known to be broken for cryptographic purposes.
>>
>> Fixes: 62a03defeabd ("PM / hibernate: Verify the consistent of e820 memory map
>>         by md5 digest")
>>
>> Tested-by: Dexuan Cui <decui@microsoft.com>
>> Reviewed-by: Dexuan Cui <decui@microsoft.com>
>> Signed-off-by: Chris von Recklinghausen <crecklin@redhat.com>
>> ---
>> v1 -> v2
>>     bump up RESTORE_MAGIC
>> v2 -> v3
>>     move embelishment from cover letter to commit comments (no code change)
>> v3 -> v4
>>     add note to comments that md5 isn't used for encryption here.
>> v4 -> v5
>>     reword comment per Simo's suggestion
>>
>>   arch/x86/power/hibernate.c | 35 +++++++++++++++++++----------------
>>   1 file changed, 19 insertions(+), 16 deletions(-)
>>
>> diff --git a/arch/x86/power/hibernate.c b/arch/x86/power/hibernate.c
>> index cd3914fc9f3d..b56172553275 100644
>> --- a/arch/x86/power/hibernate.c
>> +++ b/arch/x86/power/hibernate.c
>> @@ -55,31 +55,31 @@ int pfn_is_nosave(unsigned long pfn)
>>   }
>>   
>>   
>> -#define MD5_DIGEST_SIZE 16
>> +#define CRC32_DIGEST_SIZE 16
>>   
>>   struct restore_data_record {
>>   	unsigned long jump_address;
>>   	unsigned long jump_address_phys;
>>   	unsigned long cr3;
>>   	unsigned long magic;
>> -	u8 e820_digest[MD5_DIGEST_SIZE];
>> +	u8 e820_digest[CRC32_DIGEST_SIZE];
>>   };
>>   
>> -#if IS_BUILTIN(CONFIG_CRYPTO_MD5)
>> +#if IS_BUILTIN(CONFIG_CRYPTO_CRC32)
> Should CONFIG_CRYPTO_CRC32 be getting selected from somewhere?


Yes, presumably from the same source that sets CONFIG_CRYPTO_MD5. Also 
presumably there's value to not forcing the check if the config value is 
not set.


>
> If that is too hard because it would pull in too much of the crypto API, maybe
> using the library interface to CRC-32 (lib/crc32.c) would be a better fit?


Based on my statement above, the intent is to provide a simple drop in 
replacement for md5 so that users of FIPS mode can suspend/resume 
without any errors.

Thanks,

Chris


>
> - Eric
>