linux-kernel - Re: [PATCH 5.4 regression fix] x86/boot: Provide memzero

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Date:   Mon, 7 Oct 2019 15:00:51 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Stephan Mueller <smueller@...onox.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH 5.4 regression fix] x86/boot: Provide memzero_explicit

Hi Stephan,

On 07-10-2019 11:34, Stephan Mueller wrote:
> Am Montag, 7. Oktober 2019, 11:06:04 CEST schrieb Hans de Goede:
> 
> Hi Hans,
> 
>> Hi Stephan,
>>
>> On 07-10-2019 10:59, Stephan Mueller wrote:
>>> Am Montag, 7. Oktober 2019, 10:55:01 CEST schrieb Hans de Goede:
>>>
>>> Hi Hans,
>>>
>>>> The purgatory code now uses the shared lib/crypto/sha256.c sha256
>>>> implementation. This needs memzero_explicit, implement this.
>>>>
>>>> Reported-by: Arvind Sankar <nivedita@...m.mit.edu>
>>>> Fixes: 906a4bb97f5d ("crypto: sha256 - Use get/put_unaligned_be32 to get
>>>> input, memzero_explicit") Signed-off-by: Hans de Goede
>>>> <hdegoede@...hat.com>
>>>> ---
>>>>
>>>>    arch/x86/boot/compressed/string.c | 5 +++++
>>>>    1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/arch/x86/boot/compressed/string.c
>>>> b/arch/x86/boot/compressed/string.c index 81fc1eaa3229..511332e279fe
>>>> 100644
>>>> --- a/arch/x86/boot/compressed/string.c
>>>> +++ b/arch/x86/boot/compressed/string.c
>>>> @@ -50,6 +50,11 @@ void *memset(void *s, int c, size_t n)
>>>>
>>>>    	return s;
>>>>    
>>>>    }
>>>>
>>>> +void memzero_explicit(void *s, size_t count)
>>>> +{
>>>> +	memset(s, 0, count);
>>>
>>> May I ask how it is guaranteed that this memset is not optimized out by
>>> the
>>> compiler, e.g. for stack variables?
>>
>> The function and the caller live in different compile units, so unless
>> LTO is used this cannot happen.
> 
> Agreed in this case.
> 
> I would just be worried that this memzero_explicit implementation is assumed
> to be protected against optimization when used elsewhere since other
> implementations of memzero_explicit are provided with the goal to be protected
> against optimizations.
>>
>> Also note that the previous purgatory private (vs shared) sha256
>> implementation had:
>>
>>           /* Zeroize sensitive information. */
>>           memset(sctx, 0, sizeof(*sctx));
>>
>> In the place where the new shared 256 code uses memzero_explicit() and the
>> new shared sha256 code is the only user of the
>> arch/x86/boot/compressed/string.c memzero_explicit() implementation.
>>
>> With that all said I'm open to suggestions for improving this.
> 
> What speaks against the common memzero_explicit implementation?

Nothing, but the purgatory is a standalone binary which runs between
2 kernels when doing kexec so it cannot use the function from lib/string.c
since it is not linked against the lib/string.o object.

> If you cannot
> use it, what about adding a barrier in the memzero_explicit implementation? Or
> what about adding some compiler magic as attached to this email?

Since the purgatory code is running in a somewhat limited environment,
with not all standard headers / macros available I was afraid that the
barrier_data() from the lib/string.c implementation would not work, so
I left it out. In hindsight I should have really given it a try first as
it seems to compile fine and there are no missing symbols in
arch/x86/purgatory/purgatory.ro when using it.

So I will send out a new version with the barrier_data() added making
the arch/x86/boot/compressed/string.c implementation identical to the
lib/string.c one.

Regards,

Hans