linux-kernel - Re: [PATCH v2 4/4] virt: Add sev_secret module to expose confidential computing secrets

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Message-ID: <020a4e8b-20b6-15a0-13d3-66b232c221e3@linux.ibm.com>
Date:   Fri, 8 Oct 2021 08:51:55 +0300
From:   Dov Murik <dovmurik@...ux.ibm.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-efi@...r.kernel.org
Cc:     Borislav Petkov <bp@...e.de>, Ashish Kalra <ashish.kalra@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrew Scull <ascull@...gle.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@...ux.ibm.com>,
        Jim Cadden <jcadden@....com>,
        Daniele Buono <dbuono@...ux.vnet.ibm.com>,
        linux-coco@...ts.linux.dev, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, Dov Murik <dovmurik@...ux.ibm.com>
Subject: Re: [PATCH v2 4/4] virt: Add sev_secret module to expose confidential
 computing secrets

Thanks Dave for reviewing this.

On 07/10/2021 16:48, Dave Hansen wrote:
> On 10/6/21 11:18 PM, Dov Murik wrote:
>> +static void wipe_memory(void *addr, size_t size)
>> +{
>> +	memzero_explicit(addr, size);
>> +	clean_cache_range(addr, size);
>> +}
> 
> What's the purpose of the clean_cache_range()?  It's backed in a CLWB
> instruction on x86 which seems like an odd choice.  I guess the point is
> that the memzero_explicit() will overwrite the contents, but might have
> dirty lines in the cache.  The CLWB will ensure that the lines are
> actually written back to memory, clearing the secret out of memory.
> Without the CLWB, the secret might live in memory until the dirtied
> cachelines are written back.

Yes, that's the reason; as suggested by Andrew Scull in [1].

[1] https://lore.kernel.org/linux-coco/CADcWuH0mP+e6GxkUGN3ni_Yu0z8YTn-mo677obH+p-OFCL+wOQ@mail.gmail.com/

> 
> Could you document this, please?  It would also be nice to include some
> of this motivation in the patch that exports clean_cache_range() in the
> first place.
> 

Yes, I'll add that.


> I also think clean_cache_range() an odd choice.  If it were me, I
> probably would have just used the already-exported
> clflush_cache_range().  The practical difference between writing back
> and flushing the cachelines is basically zero.  The lines will never be
> reused.
> 

I agree that performance benefits of CLWB over CLFLUSH are negligible here
(but I have no way of measuring it).  Andrew suggested [2] that the extra
invalidation that CLFLUSH does it unnecessary.

But if we all agree that the clflush_cache_range() is OK here, I'm OK
with removing patch 1 and calling clflush_cache_range() in wipe_memory()
here.

Does anyone know of other locations in the kernel where memory is needed
to be scrubbed (zeroed and flushed) - like my wipe_memory()? Maybe there's
a standard way of doing this?


[2] https://lore.kernel.org/linux-coco/CADcWuH05vbFtJ1WYSs3d+_=TGzh-MitvAXp1__d1kGJJkvkWpQ@mail.gmail.com/


> *If* we export anything from x86 code, I think it should be something
> which is specific to the task at hand, like arch_invalidate_pmem() is.
> 
> Also, when you are modifying x86 code, including exports, it would be
> nice to include (all of) the x86 maintainers.  The relevant ones for
> this series would probably be:
> 
> X86 ARCHITECTURE (32-BIT AND 64-BIT)
> M:      Thomas Gleixner <tglx@...utronix.de>
> M:      Ingo Molnar <mingo@...hat.com>
> M:      Borislav Petkov <bp@...en8.de>
> M:      x86@...nel.org
> 
> X86 MM
> M:      Dave Hansen <dave.hansen@...ux.intel.com>
> M:      Andy Lutomirski <luto@...nel.org>
> M:      Peter Zijlstra <peterz@...radead.org>
> 
> There's also the handy dandy scripts/get_maintainer.pl to help.
> 

You're right, sorry for missing it in this round.

But even if I remove the x86 change, I'll keep you copied anyway...


-Dov