[PATCH v2 10/12] KVM: arm64: nVHE: Support CONFIG_CFI_CLANG at EL2

[Pierre-Clément Tosi]

The compiler implements kCFI by adding type information (u32) above
every function that might be indirectly called and, whenever a function
pointer is called, injects a read-and-compare of that u32 against the
value corresponding to the expected type. In case of a mismatch, a BRK
instruction gets executed. When the hypervisor triggers such an
exception in nVHE, it panics and triggers and exception return to EL1.

Therefore, teach nvhe_hyp_panic_handler() to detect kCFI errors from the
ESR and report them. If necessary, remind the user that EL2 kCFI is not
affected by CONFIG_CFI_PERMISSIVE.

Pass $(CC_FLAGS_CFI) to the compiler when building the nVHE hyp code.

Use SYM_TYPED_FUNC_START() for __pkvm_init_switch_pgd, as nVHE can't
call it directly and must use a PA function pointer from C (because it
is part of the idmap page), which would trigger a kCFI failure if the
type ID wasn't present.

Signed-off-by: Pierre-Clément Tosi <ptosi at google.com>
---
 arch/arm64/include/asm/esr.h       |  6 ++++++
 arch/arm64/kvm/handle_exit.c       | 11 +++++++++++
 arch/arm64/kvm/hyp/nvhe/Makefile   |  6 +++---
 arch/arm64/kvm/hyp/nvhe/hyp-init.S |  6 +++++-
 4 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
index 2bcf216be376..9eb9e6aa70cf 100644
--- a/arch/arm64/include/asm/esr.h
+++ b/arch/arm64/include/asm/esr.h
@@ -391,6 +391,12 @@ static inline bool esr_is_data_abort(unsigned long esr)
 	return ec == ESR_ELx_EC_DABT_LOW || ec == ESR_ELx_EC_DABT_CUR;
 }
 
+static inline bool esr_is_cfi_brk(unsigned long esr)
+{
+	return ESR_ELx_EC(esr) == ESR_ELx_EC_BRK64 &&
+	       (esr_brk_comment(esr) & ~CFI_BRK_IMM_MASK) == CFI_BRK_IMM_BASE;
+}
+
 static inline bool esr_fsc_is_translation_fault(unsigned long esr)
 {
 	/* Translation fault, level -1 */
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 0bcafb3179d6..0db23a6304ce 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -383,6 +383,15 @@ void handle_exit_early(struct kvm_vcpu *vcpu, int exception_index)
 		kvm_handle_guest_serror(vcpu, kvm_vcpu_get_esr(vcpu));
 }
 
+static void kvm_nvhe_report_cfi_failure(u64 panic_addr)
+{
+	kvm_err("nVHE hyp CFI failure at: [<%016llx>] %pB!\n", panic_addr,
+		(void *)(panic_addr + kaslr_offset()));
+
+	if (IS_ENABLED(CONFIG_CFI_PERMISSIVE))
+		kvm_err(" (CONFIG_CFI_PERMISSIVE ignored for hyp failures)\n");
+}
+
 void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr,
 					      u64 elr_virt, u64 elr_phys,
 					      u64 par, uintptr_t vcpu,
@@ -413,6 +422,8 @@ void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr,
 		else
 			kvm_err("nVHE hyp BUG at: [<%016llx>] %pB!\n", panic_addr,
 					(void *)(panic_addr + kaslr_offset()));
+	} else if (IS_ENABLED(CONFIG_CFI_CLANG) && esr_is_cfi_brk(esr)) {
+		kvm_nvhe_report_cfi_failure(panic_addr);
 	} else {
 		kvm_err("nVHE hyp panic at: [<%016llx>] %pB!\n", panic_addr,
 				(void *)(panic_addr + kaslr_offset()));
diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
index 2250253a6429..2eb915d8943f 100644
--- a/arch/arm64/kvm/hyp/nvhe/Makefile
+++ b/arch/arm64/kvm/hyp/nvhe/Makefile
@@ -89,9 +89,9 @@ quiet_cmd_hyprel = HYPREL  $@
 quiet_cmd_hypcopy = HYPCOPY $@
       cmd_hypcopy = $(OBJCOPY) --prefix-symbols=__kvm_nvhe_ $< $@
 
-# Remove ftrace, Shadow Call Stack, and CFI CFLAGS.
-# This is equivalent to the 'notrace', '__noscs', and '__nocfi' annotations.
-KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_CFI), $(KBUILD_CFLAGS))
+# Remove ftrace and Shadow Call Stack CFLAGS.
+# This is equivalent to the 'notrace' and '__noscs' annotations.
+KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))
 # Starting from 13.0.0 llvm emits SHT_REL section '.llvm.call-graph-profile'
 # when profile optimization is applied. gen-hyprel does not support SHT_REL and
 # causes a build failure. Remove profile optimization flags.
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-init.S b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
index 5a15737b4233..33fb5732ab83 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-init.S
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
@@ -5,6 +5,7 @@
  */
 
 #include <linux/arm-smccc.h>
+#include <linux/cfi_types.h>
 #include <linux/linkage.h>
 
 #include <asm/alternative.h>
@@ -268,8 +269,11 @@ SYM_CODE_END(__kvm_handle_stub_hvc)
 /*
  * void __pkvm_init_switch_pgd(struct kvm_nvhe_init_params *params,
  *                             void (*finalize_fn)(void));
+ *
+ * SYM_TYPED_FUNC_START() allows C to call this ID-mapped function indirectly
+ * using a physical pointer without triggering a kCFI failure.
  */
-SYM_FUNC_START(__pkvm_init_switch_pgd)
+SYM_TYPED_FUNC_START(__pkvm_init_switch_pgd)
 	/* Load the inputs from the VA pointer before turning the MMU off */
 	ldr	x5, [x0, #NVHE_INIT_PGD_PA]
 	ldr	x0, [x0, #NVHE_INIT_STACK_HYP_VA]
-- 
2.44.0.478.gd926399ef9-goog


-- 
Pierre