linux-kernel - Re: [PATCH v9 42/43] virt: sevguest: Add support to derive key

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Date:   Mon, 7 Feb 2022 14:08:17 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Dov Murik <dovmurik@...ux.ibm.com>, Borislav Petkov <bp@...en8.de>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        brijesh.ksingh@...il.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        Liam Merwick <liam.merwick@...cle.com>
Subject: Re: [PATCH v9 42/43] virt: sevguest: Add support to derive key



On 2/7/22 1:09 PM, Dov Murik wrote:
> 
> 
> On 07/02/2022 18:23, Brijesh Singh wrote:
>>
>>
>> On 2/7/22 2:52 AM, Borislav Petkov wrote:
>>> Those are allocated on stack, why are you clearing them?
>>
>> Yep, no need to explicitly clear it. I'll take it out in next rev.
>>
> 
> Wait, this is key material generated by PSP and passed to userspace.
> Why leave copies of it floating around kernel memory?  I thought that's
> the whole reason for these memzero_explicit() calls (maybe add a comment?).
> 


Ah, now I remember I added the memzero_explicit() to address your review 
feedback :) In that patch version, we were using the kmalloc() to store 
the response data; since then, we switched to stack. We will leak the 
key outside when the stack is converted private-> shared; I don't know 
if any of these are going to happen. I can add a comment and keep the 
memzero_explicit() call.

Boris, let me know if you are okay with it?


> As an example, in arch/x86/crypto/aesni-intel_glue.c there are two calls
> to memzero_explicit(), both on stack variables; the only reason for
> these calls (as I understand it) is to avoid some future possible leak
> of this sensitive data (keys, cipher context, etc.).  I'm sure there are
> other examples in the kernel code.
>