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Message-ID: <9503ac53-1323-eade-2863-df11a5f36b6a@amd.com>
Date: Mon, 29 Nov 2021 08:44:54 -0600
From: Brijesh Singh <brijesh.singh@....com>
To: Joerg Roedel <jroedel@...e.de>, Dave Hansen <dave.hansen@...el.com>
Cc: brijesh.singh@....com, 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>,
Vlastimil Babka <vbabka@...e.cz>,
"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 11/25/21 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).
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.
Thoughts ?
>
> This should turn an RMP fault in the kernel which is not covered in the
> uaccess exception table into a fatal error.
>
> Regards,
>
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