linux-kernel - Re: [PATCH Part1 RFC v3 14/22] x86/mm: Add support to validate memory when changing C-bit

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Message-ID: <YMetti2R6SbC0mI0@zn.tnic>
Date:   Mon, 14 Jun 2021 21:27:50 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     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,
        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 Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>, tony.luck@...el.com,
        npmccallum@...hat.com
Subject: Re: [PATCH Part1 RFC v3 14/22] x86/mm: Add support to validate
 memory when changing C-bit

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