[PATCH 0/6] Delay VERW

[Pawan Gupta]

Hi,

Legacy instruction VERW was overloaded by some processors to clear
micro-architectural CPU buffers as a mitigation of CPU bugs. This series
moves VERW execution to a later point in exit-to-user path. This is
needed because in some cases it may be possible for kernel data to be
accessed after VERW in arch_exit_to_user_mode(). Such accesses may put
data into MDS affected CPU buffers, for example:

  1. Kernel data accessed by an NMI between VERW and return-to-user can
     remain in CPU buffers (since NMI returning to kernel does not
     execute VERW to clear CPU buffers).
  2. Alyssa reported that after VERW is executed,
     CONFIG_GCC_PLUGIN_STACKLEAK=y scrubs the stack used by a system
     call. Memory accesses during stack scrubbing can move kernel stack
     contents into CPU buffers.
  3. When caller saved registers are restored after a return from
     function executing VERW, the kernel stack accesses can remain in
     CPU buffers(since they occur after VERW).

Although these cases are less practical to exploit, moving VERW closer
to ring transition reduces the attack surface.

Overview of the series:

Patch 1: Prepares VERW macros for use in asm.
Patch 2: Adds macros to 64-bit entry/exit points.
Patch 3: Adds macros to 32-bit entry/exit points.
Patch 4: Enables the new macros.
Patch 5: Cleans up C implementation.
Patch 6: Adds macro to VMenter.

Below is some performance data collected on a Skylake client compared
with previous implementation:

Baseline: v6.6-rc5

| Test               | Configuration          | Relative |
| ------------------ | ---------------------- | -------- |
| build-linux-kernel | defconfig              | 1.00     |
| hackbench          | 32 - Process           | 1.02     |
| nginx              | Short Connection - 500 | 1.01     |

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
Pawan Gupta (6):
      x86/bugs: Add asm helpers for executing VERW
      x86/entry_64: Add VERW just before userspace transition
      x86/entry_32: Add VERW just before userspace transition
      x86/bugs: Use ALTERNATIVE() instead of mds_user_clear static key
      x86/bugs: Cleanup mds_user_clear
      KVM: VMX: Move VERW closer to VMentry for MDS mitigation

 Documentation/arch/x86/mds.rst       | 20 +++++++++----------
 arch/x86/entry/entry_32.S            |  8 ++++++++
 arch/x86/entry/entry_64.S            | 14 ++++++++++++++
 arch/x86/entry/entry_64_compat.S     |  2 ++
 arch/x86/include/asm/cpufeatures.h   |  2 +-
 arch/x86/include/asm/entry-common.h  |  1 -
 arch/x86/include/asm/nospec-branch.h | 37 ++++++++++++++++++++++++------------
 arch/x86/kernel/cpu/bugs.c           | 13 +++++--------
 arch/x86/kernel/nmi.c                |  2 --
 arch/x86/kvm/vmx/vmenter.S           |  9 +++++++++
 arch/x86/kvm/vmx/vmx.c               | 10 +++++++---
 11 files changed, 80 insertions(+), 38 deletions(-)
---
base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
change-id: 20231011-delay-verw-d0474986b2c3

Best regards,
-- 
Thanks,
Pawan