linux-kernel - [PATCH v2 0/8] KVM: x86: Emulator

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Message-Id: <20220526210817.3428868-1-seanjc@google.com>
Date:   Thu, 26 May 2022 21:08:09 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        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, Kees Cook <keescook@...omium.org>,
        Robert Dinse <nanook@...imo.com>
Subject: [PATCH v2 0/8] KVM: x86: Emulator _regs fixes and cleanups

Clean up and harden the use of the x86_emulate_ctxt._regs, which is
surrounded by a fair bit of magic.  This series was prompted by bug reports
by Kees and Robert where GCC-12 flags an out-of-bounds _regs access.  The
warning is a false positive due to a now-known GCC bug, but it's cheap and
easy to harden the _regs usage, and doing so minimizing the risk of more
precisely handling 32-bit vs. 64-bit GPRs.

I didn't tag patch 2 with Fixes or Cc: stable@.  It does remedy the
GCC-12 warning, but AIUI the GCC-12 bug affects other KVM paths that
already have explicit guardrails, i.e. fixing this one case doesn't
guarantee happiness when building with CONFIG_KVM_WERROR=y, let alone
CONFIG_WERROR=y.  That said, it might be worth sending to the v5.18 stable
tree[*] as it does appear to make some configs/setups happy.

[*] KVM hasn't changed, but the warning=>error was introduced in v5.18 by
   commit e6148767825c ("Makefile: Enable -Warray-bounds").

v2:
  - Collect reviews and tests. [Vitaly, Kees, Robert]
  - Tweak patch 1's changelog to explicitly call out that dirty_regs is a
    4 byte field. [Vitaly]
  - Add Reported-by for Kees and Robert since this does technically fix a
    build breakage.
  - Use a raw literal for NR_EMULATOR_GPRS instead of VCPU_REGS_R15+1 to
    play nice with 32-bit builds. [kernel test robot]
  - Reduce the number of emulated GPRs to 8 for 32-bit builds.
  - Add and use KVM_EMULATOR_BUG_ON() to bug/kill the VM when an emulator
    bug is detected.  [Vitaly]

v1: https://lore.kernel.org/all/20220525222604.2810054-1-seanjc@google.com

Sean Christopherson (8):
  KVM: x86: Grab regs_dirty in local 'unsigned long'
  KVM: x86: Harden _regs accesses to guard against buggy input
  KVM: x86: Omit VCPU_REGS_RIP from emulator's _regs array
  KVM: x86: Use 16-bit fields to track dirty/valid emulator GPRs
  KVM: x86: Reduce the number of emulator GPRs to '8' for 32-bit KVM
  KVM: x86: Bug the VM if the emulator accesses a non-existent GPR
  KVM: x86: Bug the VM if the emulator generates a bogus exception
    vector
  KVM: x86: Bug the VM on an out-of-bounds data read

 arch/x86/kvm/emulate.c     | 26 ++++++++++++++++++++------
 arch/x86/kvm/kvm_emulate.h | 28 +++++++++++++++++++++++++---
 arch/x86/kvm/x86.c         |  9 +++++++++
 3 files changed, 54 insertions(+), 9 deletions(-)

base-commit: 90bde5bea810d766e7046bf5884f2ccf76dd78e9
-- 
2.36.1.255.ge46751e96f-goog