Date:   Tue, 16 Nov 2021 14:02:06 +0100
From:   Joerg Roedel <jroedel@suse.de>
To:     Sean Christopherson <seanjc@google.com>
Cc:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Peter Gonda <pgonda@google.com>,
        Brijesh Singh <brijesh.singh@amd.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org,
        linux-crypto@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Michael Roth <michael.roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support
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References: <20210820155918.7518-1-brijesh.singh@amd.com>
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 <YY6z5/0uGJmlMuM6@zn.tnic>
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On Mon, Nov 15, 2021 at 06:26:16PM +0000, Sean Christopherson wrote:
> No, because as Andy pointed out, host userspace must already guard against a bad
> GPA, i.e. this is just a variant of the guest telling the host to DMA to a GPA
> that is completely bogus.  The shared vs. private behavior just means that when
> host userspace is doing a GPA=>HVA lookup, it needs to incorporate the "shared"
> state of the GPA.  If the host goes and DMAs into the completely wrong HVA=>PFN,
> then that is a host bug; that the bug happened to be exploited by a buggy/malicious
> guest doesn't change the fact that the host messed up.

The thing is that the usual checking mechanisms can't be applied to
guest-private pages. For user-space the GPA is valid if it fits into the
guest memory layout user-space set up before. But whether a page is
shared or private is the guests business. And without an expensive
reporting/query mechanism user-space doesn't have the information to do
the check.

A mechanism to lock pages to shared is also needed, and that creates the
next problems:

	* Who can release the lock, only the process which created it or
	  anyone who has the memory mapped?

	* What happens when a process has locked guest regions and then
	  dies with SIGSEGV, will its locks on guest memory be released
	  stay around forever?

And this is only what comes to mind immediatly, I sure there are more
problematic details in such an interface.

Regards,

-- 
J?rg R?del
jroedel@suse.de

SUSE Software Solutions Germany GmbH
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