linux-kernel - Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

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Message-ID: <d673f082-9023-dafb-e42e-eab32a3ddd0c@intel.com>
Date:   Mon, 22 Nov 2021 10:30:17 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Brijesh Singh <brijesh.singh@....com>,
        Peter Gonda <pgonda@...gle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.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>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.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>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support

On 11/22/21 7:23 AM, Brijesh Singh wrote:
> Thank you for starting the thread; based on the discussion, I am keeping
> the current implementation as-is and *not* going with the auto
> conversion from private to shared. To summarize what we are doing in the
> current SNP series:
> 
> - If userspace accesses guest private memory, it gets SIGBUS.
> - If kernel accesses[*] guest private memory, it does panic.

There's a subtlety here, though.  There are really three *different*
kinds of kernel accesses that matter:

1. Kernel bugs.  Kernel goes off and touches some guest private memory
   when it didn't mean to.  Say, it runs off the end of a slab page and
   runs into a guest page.  panic() is expected here.
2. Kernel accesses guest private memory via a userspace mapping, in a
   place where it is known to be accessing userspace and is prepared to
   fault.  copy_to_user() is the most straightforward example.  Kernel
   must *not* panic().  Returning an error to the syscall is a good
   way to handle these (if in a syscall).
3. Kernel accesses guest private memory via a kernel mapping.  This one
   is tricky.  These probably *do* result in a panic() today, but
   ideally shouldn't.

Could you explicitly clarify what the current behavior is?