linux-kernel - [PATCH v2 0/5] KVM: x86/mmu: Don't synthesize triple fault on bad root

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Message-ID: <20230729005200.1057358-1-seanjc@google.com>
Date:   Fri, 28 Jul 2023 17:51:55 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Reima Ishii <ishiir@...cc.u-tokyo.ac.jp>
Subject: [PATCH v2 0/5] KVM: x86/mmu: Don't synthesize triple fault on bad root

Rework the handling of !visible guest root gfns to wait until the guest
actually tries to access memory before synthesizing a fault.  KVM currently
just immediately synthesizes triple fault, which causes problems for nVMX
and nSVM as immediately injecting a fault causes KVM to try and forward the
fault to L1 (as a VM-Exit) before completing nested VM-Enter, e.g. if L1
runs L2 with a "bad" nested TDP root.

To get around the conundrum of not wanting to shadow garbage, load a dummy
root, backed by the zero page, into CR3/EPTP/nCR3, and then inject an
appropriate page fault when the guest (likely) hits a !PRESENT fault.

Note, KVM's behavior is still not strictly correct with respect to x86
architecture, the primary goal is purely to prevent triggering KVM's WARN
at will.  No real world guest intentionally loads CR3 (or EPTP or nCR3)
with a GPA that points at MMIO and expects it to work (and KVM has a long
and storied history of punting on emulated MMIO corner cases).

I didn't Cc any of this for stable because syzkaller is really the only
thing that I expect to care, and the whole dummy root thing isn't exactly
risk free.  If someone _really_ wants to squash the WARN in LTS kernels,
the way to do that would be to exempt triple fault shutdown VM-Exits from
the sanity checks in nVMX and nSVM, i.e. sweep the problem under the rug.

I have a KUT test for this that'll I'll post next week (I said that about
v1 and then forgot).

v2: 
 - Finish writing the changelog for patch 3. [Yu]
 - Use KVM_REQ_MMU_FREE_OBSOLETE_ROOTS instead of directly unloading
   all roots. [Yu]

v1: https://lore.kernel.org/all/20230722012350.2371049-1-seanjc@google.com

Sean Christopherson (5):
  KVM: x86/mmu: Add helper to convert root hpa to shadow page
  KVM: x86/mmu: Harden new PGD against roots without shadow pages
  KVM: x86/mmu: Harden TDP MMU iteration against root w/o shadow page
  KVM: x86/mmu: Disallow guest from using !visible slots for page tables
  KVM: x86/mmu: Use dummy root, backed by zero page, for !visible guest
    roots

 arch/x86/kvm/mmu/mmu.c          | 94 ++++++++++++++++++---------------
 arch/x86/kvm/mmu/mmu_internal.h | 10 ++++
 arch/x86/kvm/mmu/paging_tmpl.h  | 18 ++++++-
 arch/x86/kvm/mmu/spte.h         | 12 +++++
 arch/x86/kvm/mmu/tdp_iter.c     | 11 ++--
 arch/x86/kvm/mmu/tdp_mmu.c      |  2 +-
 6 files changed, 98 insertions(+), 49 deletions(-)

base-commit: fdf0eaf11452d72945af31804e2a1048ee1b574c
-- 
2.41.0.487.g6d72f3e995-goog