linux-kernel - Re: [PATCH v2 00/17] KVM: x86/mmu: Lots of bug fixes

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Date:   Fri, 5 Mar 2021 18:53:11 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 00/17] KVM: x86/mmu: Lots of bug fixes

On 05/03/21 02:10, Sean Christopherson wrote:
> Fix nested NPT (nSVM) with 32-bit L1 and SME with shadow paging, which
> are completely broken.  Opportunistically fix theoretical bugs related to
> prematurely reloading/unloading the MMU.
> 
> If nNPT is enabled, L1 can crash the host simply by using 32-bit NPT to
> trigger a null pointer dereference on pae_root.
> 
> SME with shadow paging (including nNPT) fails to set the C-bit in the
> shadow pages that don't go through standard MMU flows (PDPTPRs and the
> PML4 used by nNPT to shadow legacy NPT).  It also failes to account for
> CR3[63:32], and thus the C-bit, being ignored outside of 64-bit mode.
> 
> Patches 01 and 02 fix the null pointer bugs.
> 
> Patches 03-09 fix mostly-benign related memory leaks.
> 
> Patches 10-12 fix the SME shadow paging bugs, which are also what led me to
> the nNPT null pointer bugs.
> 
> Patches 13 and 14 fix theoretical bugs with PTP_SWITCH and INVPCID that
> I found when auditing flows that touch the MMU context.
> 
> Patches 14-17 do additional clean up to hopefully make it harder to
> introduce bugs in the future.
> 
> On the plus side, I finally understand why KVM supports shadowing 2-level
> page tables with 4-level page tables...
> 
> Based on kvm/queue, commit fe5f0041c026 ("KVM/SVM: Move vmenter.S exception
> fixups out of line").  The null pointer fixes cherry-pick cleanly onto
> kvm/master, haven't tried the other bug fixes (I doubt they're worth
> backporting even though I tagged 'em with stable).
> 
> v2:
>    - Collect a review from Ben (did not include his review of patch 03
>      since the patch and its direct dependencies were changed).
>    - Move pae_root and lm_root allocation to a separate helper to avoid
>      sleeping via get_zeroed_page() while holding mmu_lock.
>    - Add a patch to grab 'mmu' in a local variable.
>    - Remove the BUILD_BUG_ON() in make_mmu_pages_available() since the
>      final check wouldn't actually guarnatee 4 pages were "available".
>      Instead, add a comment about the limit being soft.
> 
> v1:
>    - https://lkml.kernel.org/r/20210302184540.2829328-1-seanjc@google.com
>   
> Sean Christopherson (17):
>    KVM: nSVM: Set the shadow root level to the TDP level for nested NPT
>    KVM: x86/mmu: Alloc page for PDPTEs when shadowing 32-bit NPT with
>      64-bit
>    KVM: x86/mmu: Capture 'mmu' in a local variable when allocating roots
>    KVM: x86/mmu: Allocate the lm_root before allocating PAE roots
>    KVM: x86/mmu: Allocate pae_root and lm_root pages in dedicated helper
>    KVM: x86/mmu: Ensure MMU pages are available when allocating roots
>    KVM: x86/mmu: Check PDPTRs before allocating PAE roots
>    KVM: x86/mmu: Fix and unconditionally enable WARNs to detect PAE leaks
>    KVM: x86/mmu: Use '0' as the one and only value for an invalid PAE
>      root
>    KVM: x86/mmu: Set the C-bit in the PDPTRs and LM pseudo-PDPTRs
>    KVM: x86/mmu: Mark the PAE roots as decrypted for shadow paging
>    KVM: SVM: Don't strip the C-bit from CR2 on #PF interception
>    KVM: nVMX: Defer the MMU reload to the normal path on an EPTP switch
>    KVM: x86: Defer the MMU unload to the normal path on an global INVPCID
>    KVM: x86/mmu: Unexport MMU load/unload functions
>    KVM: x86/mmu: Sync roots after MMU load iff load as successful
>    KVM: x86/mmu: WARN on NULL pae_root or lm_root, or bad shadow root
>      level
> 
>   arch/x86/include/asm/kvm_host.h |   3 -
>   arch/x86/kvm/mmu.h              |   4 +
>   arch/x86/kvm/mmu/mmu.c          | 273 ++++++++++++++++++++------------
>   arch/x86/kvm/mmu/tdp_mmu.c      |  23 +--
>   arch/x86/kvm/svm/svm.c          |   9 +-
>   arch/x86/kvm/vmx/nested.c       |   9 +-
>   arch/x86/kvm/x86.c              |   2 +-
>   7 files changed, 192 insertions(+), 131 deletions(-)
> 

Queued all except 9 and 11, thanks.

Paolo