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

Sean Christopherson
2022-05-26T21:24:24Z

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