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

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Date:   Mon, 29 Nov 2021 15:58:33 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Brijesh Singh <brijesh.singh@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Peter Gonda <pgonda@...gle.com>, 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>,
        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>,
        "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 11/29/21 15:44, Brijesh Singh wrote:
> 
> 
> On 11/25/21 4:05 AM, Joerg Roedel wrote:
>> On Wed, Nov 24, 2021 at 09:48:14AM -0800, Dave Hansen wrote:
>>> That covers things like copy_from_user().  It does not account for
>>> things where kernel mappings are used, like where a
>>> get_user_pages()/kmap() is in play.
>>
>> The kmap case is guarded by KVM code, which locks the page first so that
>> the guest can't change the page state, then checks the page state, and
>> if it is shared does the kmap and the access.
> 
> 
> The KVM use-case is well covered in the series, but I believe Dave is
> highlighting what if the access happens outside of the KVM driver (such as a
> ptrace() or others).

AFAIU ptrace() is a scenario where the userspace mapping is being gup-ped,
not a kernel page being kmap()ed?

> One possible approach to fix this is to enlighten the kmap/unmap().
> Basically, move the per page locking mechanism used by the KVM in the
> arch-specific code and have kmap/kunmap() call the arch hooks. The arch
> hooks will do this:
> 
> Before the map, check whether the page is added as a shared in the RMP
> table. If not shared, then error.
> Acquire a per-page map_lock.
> Release the per-page map_lock on the kunmap().
> 
> The current patch set provides helpers to change the page from private to
> shared. Enhance the helpers to check for the per-page map_lock, if the
> map_lock is held then do not allow changing the page from shared to private.

That could work for the kmap() context.
What to do for the userspace context (host userspace)?
- shared->private transition - page has to be unmapped from all userspace,
elevated refcount (gup() in progress) can block this unmap until it goes
away - could be doable
- still, what to do if host userspace then tries to access the unmapped
page? SIGSEGV instead of SIGBUS and it can recover?



> Thoughts ?
> 
>>
>> This should turn an RMP fault in the kernel which is not covered in the
>> uaccess exception table into a fatal error.
>>
>> Regards,
>>