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Date:   Mon, 14 Jun 2021 21:27:50 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Brijesh Singh <brijesh.singh@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org,
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        linux-crypto@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
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        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
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        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
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        David Rientjes <rientjes@google.com>, tony.luck@intel.com,
        npmccallum@redhat.com
Subject: Re: [PATCH Part1 RFC v3 14/22] x86/mm: Add support to validate
 memory when changing C-bit
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On Mon, Jun 14, 2021 at 08:05:51AM -0500, Brijesh Singh wrote:
> Guest OS depend on the hypervisor to assist in this operation. The loop
> will terminate only after the hypervisor completes the requested
> operation. Guest is not protecting itself from DoS type of attack. A
> guest should not proceed until hypervisor performs the request page
> state change in the RMP table.

Some of that could be in a comment over that loop, so that it is clear
what the guest strategy is.

> Let me understand, are you saying that hypervisor could trick us into
> believing that page state change completed without actually changing it ?

Nah, I'm just saying that you should verify those ->cur_entry and
->end_entry values.

Of course the guest doesn't protect itself against DoS types of attacks
but this function page_state_vmgexit() here could save ->cur_entry
and ->end_entry on function entry and then compare it each time the
hypercall returns to make sure HV is not doing some shenanigans with
the entries range or even has a bug or so. I.e., it has not changed
->end_entry or ->cur_entry is not going backwards into the buffer.

I know, if uncaught here, it probably will explode later but a cheap
sanity check like that doesn't hurt to have just in case.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette