linux-kernel - Re: [PATCH Part2 RFC v2 10/37] x86/fault: Add support to handle the RMP fault for kernel address

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Message-ID: <af581395-1322-a668-d5f3-3784bbfd6c9b@amd.com>
Date:   Tue, 4 May 2021 07:31:20 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, jroedel@...e.de,
        Thomas.Lendacky@....com, pbonzini@...hat.com, mingo@...hat.com,
        rientjes@...gle.com, seanjc@...gle.com, peterz@...radead.org,
        hpa@...or.com, tony.luck@...el.com
Subject: Re: [PATCH Part2 RFC v2 10/37] x86/fault: Add support to handle the
 RMP fault for kernel address

On 5/3/21 2:43 PM, Dave Hansen wrote:
> On 5/3/21 12:41 PM, Brijesh Singh wrote:
>> Sure, I will look into all the drivers which do a walk plus kmap to make
>> sure that they fail instead of going into the fault path. Should I drop
>> this patch or keep it just in the case we miss something?
> I think you should drop it, and just ensure that the existing page fault
> oops code can produce a coherent, descriptive error message about what
> went wrong.

A malicious guest could still trick the host into accessing a guest
private page unless we make sure that host kernel *never* does kmap() on
GPA. The example I was thinking is:

1. Guest provides a GPA to host.

2. Host queries the RMP table and finds that GPA is shared and allows
the kmap() to happen.

3. Guest later changes the page to private.

4. Host write to mapped address will trigger a page-fault.

KVM provides kvm_map_gfn(), kvm_vcpu_map() to map a GPA; these APIs will
no longer be safe to be used. In addition, some shared pages are
registered once by the guest and KVM updates the contents of the page on
vcpu enter (e.g, CPU steal time).

IMHO, we should add the RMP table check before kmap'ing GPA but still
keep this patch to mitigate the cases where a malicious guest changes
the page state after the kmap().

-Brijesh