[v3,00/12] KVM: arm64: Add support for hypervisor kCFI

Message

Pierre-Clément Tosi May 10, 2024, 11:26 a.m. UTC

CONFIG_CFI_CLANG ("kernel Control Flow Integrity") makes the compiler inject
runtime type checks before any indirect function call. On AArch64, it generates
a BRK instruction to be executed on type mismatch and encodes the indices of the
registers holding the branch target and expected type in the immediate of the
instruction. As a result, a synchronous exception gets triggered on kCFI failure
and the fault handler can retrieve the immediate (and indices) from ESR_ELx.

This feature has been supported at EL1 ("host") since it was introduced by
b26e484b8bb3 ("arm64: Add CFI error handling"), where cfi_handler() decodes
ESR_EL1, giving informative panic messages such as

  [   21.885179] CFI failure at lkdtm_indirect_call+0x2c/0x44 [lkdtm]
  (target: lkdtm_increment_int+0x0/0x1c [lkdtm]; expected type: 0x7e0c52a)
  [   21.886593] Internal error: Oops - CFI: 0 [#1] PREEMPT SMP

However, it is not or only partially supported at EL2: in nVHE (or pKVM),
CONFIG_CFI_CLANG gets filtered out at build time, preventing the compiler from
injecting the checks. In VHE, EL2 code gets compiled with the checks but the
handlers in VBAR_EL2 are not aware of kCFI and will produce a generic and
not-so-helpful panic message such as

  [   36.456088][  T200] Kernel panic - not syncing: HYP panic:
  [   36.456088][  T200] PS:204003c9 PC:ffffffc080092310 ESR:f2008228
  [   36.456088][  T200] FAR:0000000081a50000 HPFAR:000000000081a500 PAR:1de7ec7edbadc0de
  [   36.456088][  T200] VCPU:00000000e189c7cf

To address this,

- [01/12] fixes an existing bug where the ELR_EL2 was getting clobbered on
  synchronous exceptions, causing the wrong "PC" to be reported by
  nvhe_hyp_panic_handler() or __hyp_call_panic(). This is particularly limiting
  for kCFI, as it would mask the location of the failed type check.
- [02/12] & [03/12] (resp.) fix and improve __pkvm_init_switch_pgd for kCFI
- [04/12] to [06/12] prepare nVHE for CONFIG_CFI_CLANG and [10/12] enables it
- [12/12] improves kCFI error messages by saving then parsing the CPU context
- [09/12] adds a kCFI test module for VHE and [11/12] extends it to nVHE & pKVM

As a result, an informative kCFI panic message is printed by or on behalf of EL2
giving the expected type and target address (possibly resolved to a symbol) for
VHE, nVHE, and pKVM (iff CONFIG_NVHE_EL2_DEBUG=y).

Note that kCFI errors remain fatal at EL2, even when CONFIG_CFI_PERMISSIVE=y.

Change in v3:
  - Reworked the commit message of [04/12]
  - (no code changes since v2, questions on v1 remain open)

Changes in v2:
  - Added 2 commits implementing a test module for hyp kCFI;
  - For __kvm_hyp_host_vector, dropped changes to the sync EL2t handler and kept
    the SP overflow checks;
  - Used the names __guest_exit_restore_elr_and_panic, esr_brk_comment;
  - Documented the use of SYM_TYPED_FUNC_START for __pkvm_init_switch_pgd;
  - Fixed or clarified commit messages.

Pierre-Clément Tosi (12):
  KVM: arm64: Fix clobbered ELR in sync abort/SError
  KVM: arm64: Fix __pkvm_init_switch_pgd C signature
  KVM: arm64: Pass pointer to __pkvm_init_switch_pgd
  KVM: arm64: nVHE: Remove __guest_exit_panic path
  KVM: arm64: nVHE: Add EL2h sync exception handler
  KVM: arm64: nVHE: gen-hyprel: Skip R_AARCH64_ABS32
  KVM: arm64: VHE: Mark __hyp_call_panic __noreturn
  arm64: Move esr_comment() to <asm/esr.h>
  KVM: arm64: VHE: Add test module for hyp kCFI
  KVM: arm64: nVHE: Support CONFIG_CFI_CLANG at EL2
  KVM: arm64: nVHE: Support test module for hyp kCFI
  KVM: arm64: Improve CONFIG_CFI_CLANG error message

 arch/arm64/include/asm/esr.h            |  11 ++
 arch/arm64/include/asm/kvm_asm.h        |   3 +
 arch/arm64/include/asm/kvm_cfi.h        |  38 +++++++
 arch/arm64/include/asm/kvm_hyp.h        |   4 +-
 arch/arm64/kernel/asm-offsets.c         |   1 +
 arch/arm64/kernel/debug-monitors.c      |   4 +-
 arch/arm64/kernel/traps.c               |   8 +-
 arch/arm64/kvm/Kconfig                  |  20 ++++
 arch/arm64/kvm/Makefile                 |   3 +
 arch/arm64/kvm/handle_exit.c            |  39 ++++++-
 arch/arm64/kvm/hyp/cfi.c                |  37 +++++++
 arch/arm64/kvm/hyp/entry.S              |  43 +++++++-
 arch/arm64/kvm/hyp/hyp-entry.S          |   4 +-
 arch/arm64/kvm/hyp/include/hyp/cfi.h    |  47 +++++++++
 arch/arm64/kvm/hyp/include/hyp/switch.h |   5 +-
 arch/arm64/kvm/hyp/nvhe/Makefile        |   7 +-
 arch/arm64/kvm/hyp/nvhe/gen-hyprel.c    |   6 ++
 arch/arm64/kvm/hyp/nvhe/host.S          |  20 ++--
 arch/arm64/kvm/hyp/nvhe/hyp-init.S      |  18 +++-
 arch/arm64/kvm/hyp/nvhe/hyp-main.c      |  19 ++++
 arch/arm64/kvm/hyp/nvhe/setup.c         |   6 +-
 arch/arm64/kvm/hyp/nvhe/switch.c        |   7 ++
 arch/arm64/kvm/hyp/vhe/Makefile         |   1 +
 arch/arm64/kvm/hyp/vhe/switch.c         |  34 +++++-
 arch/arm64/kvm/hyp_cfi_test.c           |  75 +++++++++++++
 arch/arm64/kvm/hyp_cfi_test_module.c    | 135 ++++++++++++++++++++++++
 26 files changed, 553 insertions(+), 42 deletions(-)
 create mode 100644 arch/arm64/include/asm/kvm_cfi.h
 create mode 100644 arch/arm64/kvm/hyp/cfi.c
 create mode 100644 arch/arm64/kvm/hyp/include/hyp/cfi.h
 create mode 100644 arch/arm64/kvm/hyp_cfi_test.c
 create mode 100644 arch/arm64/kvm/hyp_cfi_test_module.c