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.
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: Simplify __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
--
2.44.0.478.gd926399ef9-goog
--
Pierre
