KVM: arm64: nVHE: Support CONFIG_CFI_CLANG at EL2

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 <[email protected]>
Acked-by: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Oliver Upton <[email protected]>

Author: ptosi

arch/arm64/kvm/handle_exit.c

@@ -417,6 +417,14 @@ static void print_nvhe_hyp_panic(const char *name, u64 panic_addr)
 		(void *)(panic_addr + kaslr_offset()));
 }
 
+static void kvm_nvhe_report_cfi_failure(u64 panic_addr)
+{
+	print_nvhe_hyp_panic("CFI failure", panic_addr);
+
+	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,
@@ -446,6 +454,8 @@ void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr,
 			kvm_err("nVHE hyp BUG at: %s:%u!\n", file, line);
 		else
 			print_nvhe_hyp_panic("BUG", panic_addr);
+	} else if (IS_ENABLED(CONFIG_CFI_CLANG) && esr_is_cfi_brk(esr)) {
+		kvm_nvhe_report_cfi_failure(panic_addr);
 	} else {
 		print_nvhe_hyp_panic("panic", panic_addr);
 	}

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.

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(phys_addr_t pgd, unsigned long sp,
  *                             void (*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)
 	/* Turn the MMU off */
 	pre_disable_mmu_workaround
 	mrs	x3, sctlr_el2