linux-kernel - Re: [PATCH v8 36/40] x86/sev: Provide support for SNP guest request NAEs

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Message-ID: <87d4999a-14cc-5070-4f03-001dd5f1d2b1@amd.com>
Date:   Thu, 27 Jan 2022 11:02:13 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     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>,
        Dov Murik <dovmurik@...ux.ibm.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>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 36/40] x86/sev: Provide support for SNP guest request
 NAEs



On 1/27/22 10:21 AM, Borislav Petkov wrote:
> On Fri, Dec 10, 2021 at 09:43:28AM -0600, Brijesh Singh wrote:
>> Version 2 of GHCB specification provides SNP_GUEST_REQUEST and
>> SNP_EXT_GUEST_REQUEST NAE that can be used by the SNP guest to communicate
>> with the PSP.
>>
>> While at it, add a snp_issue_guest_request() helper that can be used by
> 
> Not "that can" but "that will".
> 
Noted.

>>   
>> +/* Guest message request error code */
>> +#define SNP_GUEST_REQ_INVALID_LEN	BIT_ULL(32)
> 
> SZ_4G is more descriptive, perhaps...
> 

I am okay with using SZ_4G but per the spec they don't spell that its 4G 
size. It says bit 32 will should be set on error.



>> +
>> +	ret = sev_es_ghcb_hv_call(ghcb, true, NULL, exit_code, input->req_gpa, input->resp_gpa);
> 					      ^^^^^
> 
> That's ctxt which is accessed without a NULL check in
> verify_exception_info().
> 
> Why aren't you allocating a ctxt on stack like the other callers do?

Typically the sev_es_ghcb_hv_handler() is called from #VC handler, which 
provides the context structure. But in this and PSC case, the caller is 
not a #VC handler, so we don't have a context structure. But as you 
pointed, we could allocate context structure on the stack and pass it 
down so that verify_exception_info() does not cause a panic with NULL 
deference (when HV violates the spec and inject exception while handling 
this NAE).

thanks