Re: [PATCH v2 0/8] KVM: x86: Emulator _regs fixes and cleanups

Paolo Bonzini
2022-06-08T12:23:20Z

On 5/26/22 23:08, Sean Christopherson wrote:
> 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

Queued, thanks.

Paolo