[PATCH v6 0/6] Delay VERW

[Pawan Gupta]

v6:
- Simplify FB_CLEAR checks in vmx_update_fb_clear_dis() and the comment. (Sean)

v5: https://lore.kernel.org/r/20240111-delay-verw-v5-0-a3b234933ea6@xxxxxxxxxxxxxxx
- Added comment to SYM_CODE_START_NOALIGN(mds_verw_sel) explaining VERW
  operand is in code segment so that VERW at works with KPTI. (Josh/Borislav).
- Fixed changelog for patch 1/6. (Borislav)
- Clarify CLEAR_CPU_BUFFERS macro documentation. (Josh)
- KVM: Move the check to skip FB_CLEAR_CTRL optimization for guests when
  X86_FEATURE_CLEAR_CPU_BUF is set. (Josh)
- Rebased to v6.7

v4: https://lore.kernel.org/all/20231027-delay-verw-v4-0-9a3622d4bcf7@xxxxxxxxxxxxxxx/
- Fill unused part of mds_verw_sel cacheline with int3. (Andrew)
- Fix the formatting in documentation (0-day).
- s/inspite/in spite/ (Sean).
- Explicitly skip FB_CLEAR optimization when MDS affected (Sean).

v3: https://lore.kernel.org/r/20231025-delay-verw-v3-0-52663677ee35@xxxxxxxxxxxxxxx
- Use .entry.text section for VERW memory operand. (Andrew/PeterZ)
- Fix the duplicate header inclusion. (Chao)

v2: https://lore.kernel.org/r/20231024-delay-verw-v2-0-f1881340c807@xxxxxxxxxxxxxxx
- Removed the extra EXEC_VERW macro layers. (Sean)
- Move NOPL before VERW. (Sean)
- s/USER_CLEAR_CPU_BUFFERS/CLEAR_CPU_BUFFERS/. (Josh/Dave)
- Removed the comments before CLEAR_CPU_BUFFERS. (Josh)
- Remove CLEAR_CPU_BUFFERS from NMI returning to kernel and document the
  reason. (Josh/Dave)
- Reformat comment in md_clear_update_mitigation(). (Josh)
- Squash "x86/bugs: Cleanup mds_user_clear" patch. (Nikolay)
- s/GUEST_CLEAR_CPU_BUFFERS/CLEAR_CPU_BUFFERS/. (Josh)
- Added a patch from Sean to use CFLAGS.CF for VMLAUNCH/VMRESUME
  selection. This facilitates a single CLEAR_CPU_BUFFERS location for both
  VMLAUNCH and VMRESUME. (Sean)

v1: https://lore.kernel.org/r/20231020-delay-verw-v1-0-cff54096326d@xxxxxxxxxxxxxxx

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: Uses CFLAGS.CF for VMLAUNCH/VMRESUME selection.
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          | v1   | v3   |
| ------------------ | ---------------------- | ---- | ---- |
| build-linux-kernel | defconfig              | 1.00 | 1.00 |
| hackbench          | 32 - Process           | 1.02 | 1.06 |
| nginx              | Short Connection - 500 | 1.01 | 1.04 |

Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx
Cc: Alyssa Milburn <alyssa.milburn@xxxxxxxxxxxxxxx>
Cc: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
Cc: antonio.gomez.iglesias@xxxxxxxxxxxxxxx
To: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
To: Ingo Molnar <mingo@xxxxxxxxxx>
To: Borislav Petkov <bp@xxxxxxxxx>
To: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
To: x86@xxxxxxxxxx
To: "H. Peter Anvin" <hpa@xxxxxxxxx>
To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
To: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
To: Andy Lutomirski <luto@xxxxxxxxxx>
To: Jonathan Corbet <corbet@xxxxxxx>
To: Sean Christopherson <seanjc@xxxxxxxxxx>
To: Paolo Bonzini <pbonzini@xxxxxxxxxx>
To: tony.luck@xxxxxxxxx
To: ak@xxxxxxxxxxxxxxx
To: tim.c.chen@xxxxxxxxxxxxxxx
To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
To: Nikolay Borisov <nik.borisov@xxxxxxxx>

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
Pawan Gupta (5):
      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
      KVM: VMX: Move VERW closer to VMentry for MDS mitigation

Sean Christopherson (1):
      KVM: VMX: Use BT+JNC, i.e. EFLAGS.CF to select VMRESUME vs. VMLAUNCH

 Documentation/arch/x86/mds.rst       | 38 +++++++++++++++++++++++++-----------
 arch/x86/entry/entry.S               | 22 +++++++++++++++++++++
 arch/x86/entry/entry_32.S            |  3 +++
 arch/x86/entry/entry_64.S            | 11 +++++++++++
 arch/x86/entry/entry_64_compat.S     |  1 +
 arch/x86/include/asm/cpufeatures.h   |  2 +-
 arch/x86/include/asm/entry-common.h  |  1 -
 arch/x86/include/asm/nospec-branch.h | 27 +++++++++++++------------
 arch/x86/kernel/cpu/bugs.c           | 15 ++++++--------
 arch/x86/kernel/nmi.c                |  3 ---
 arch/x86/kvm/vmx/run_flags.h         |  7 +++++--
 arch/x86/kvm/vmx/vmenter.S           |  9 ++++++---
 arch/x86/kvm/vmx/vmx.c               | 20 +++++++++++++++----
 13 files changed, 113 insertions(+), 46 deletions(-)
---
base-commit: 0dd3ee31125508cd67f7e7172247f05b7fd1753a
change-id: 20231011-delay-verw-d0474986b2c3

Best regards,
-- 
Thanks,
Pawan