[RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

Sami Tolvanen
2022-05-13T20:24:10Z

With CONFIG_CFI_CLANG, the compiler injects a type preamble
immediately before each function and a check to validate the target
function type before indirect calls:

; type preamble
__cfi_function:
int3
int3
mov <id>, %eax
int3
int3
function:
...
; indirect call check
cmpl    <id>, -6(%r11)
je .Ltmp1
ud2
.Ltmp1:
call __x86_indirect_thunk_r11

Define the __CFI_TYPE helper macro for manual type annotations in
assembly code, add error handling for the CFI ud2 traps, and allow
CONFIG_CFI_CLANG to be selected on x86_64.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
arch/x86/Kconfig | 2 ++
arch/x86/include/asm/linkage.h | 12 +++++++
arch/x86/kernel/traps.c | 60 +++++++++++++++++++++++++++++++++-
3 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 4bed3abf444d..2e73d0792d48 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -108,6 +108,8 @@ config X86
select ARCH_SUPPORTS_PAGE_TABLE_CHECK if X86_64
select ARCH_SUPPORTS_NUMA_BALANCING if X86_64
select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP if NR_CPUS <= 4096
+ select ARCH_SUPPORTS_CFI_CLANG if X86_64
+ select ARCH_USES_CFI_TRAPS if X86_64 && CFI_CLANG
select ARCH_SUPPORTS_LTO_CLANG
select ARCH_SUPPORTS_LTO_CLANG_THIN
select ARCH_USE_BUILTIN_BSWAP
diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
index 85865f1645bd..0ee4a0af3974 100644
--- a/arch/x86/include/asm/linkage.h
+++ b/arch/x86/include/asm/linkage.h
@@ -25,6 +25,18 @@
#define RET ret
#endif

+#ifdef CONFIG_CFI_CLANG
+#define __CFI_TYPE(name) \
+ .fill 7, 1, 0xCC ASM_NL \
+ SYM_START(__cfi_##name, SYM_L_LOCAL, SYM_A_NONE) \
+ int3 ASM_NL \
+ int3 ASM_NL \
+ mov __kcfi_typeid_##name, %eax ASM_NL \
+ int3 ASM_NL \
+ int3 ASM_NL \
+ SYM_FUNC_END(__cfi_##name)
+#endif
+
#else /* __ASSEMBLY__ */

#ifdef CONFIG_SLS
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 1563fb995005..320e257eb4be 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -40,6 +40,7 @@
#include <linux/hardirq.h>
#include <linux/atomic.h>
#include <linux/ioasid.h>
+#include <linux/cfi.h>

#include <asm/stacktrace.h>
#include <asm/processor.h>
@@ -295,6 +296,62 @@ static inline void handle_invalid_op(struct pt_regs *regs)
ILL_ILLOPN, error_get_trap_addr(regs));
}

+#ifdef CONFIG_CFI_CLANG
+static void decode_cfi_insn(struct pt_regs *regs, unsigned long *target,
+ unsigned long *type)
+{
+ char buffer[MAX_INSN_SIZE];
+ struct insn insn;
+ int offset;
+
+ *target = *type = 0;
+
+ /*
+ * The compiler generates the following instruction sequence
+ * for indirect call checks:
+ *
+ *   cmpl    <id>, -6(%reg) ; 7 bytes
+ * je .Ltmp1 ; 2 bytes
+ * ud2 ; <- addr
+ * .Ltmp1:
+ *
+ * Both the type and the target address can be decoded from the
+ * cmpl instruction.
+ */
+ if (copy_from_kernel_nofault(buffer, (void *)regs->ip - 9, MAX_INSN_SIZE))
+ return;
+ if (insn_decode_kernel(&insn, buffer))
+ return;
+ if (insn.opcode.value != 0x81 || X86_MODRM_REG(insn.modrm.value) != 7)
+ return;
+
+ *type = insn.immediate.value;
+
+ offset = insn_get_modrm_rm_off(&insn, regs);
+ if (offset < 0)
+ return;
+
+ *target = *(unsigned long *)((void *)regs + offset);
+}
+
+static enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
+{
+ if (is_cfi_trap(regs->ip)) {
+ unsigned long target, type;
+
+ decode_cfi_insn(regs, &target, &type);
+ return report_cfi_failure(regs, regs->ip, target, type);
+ }
+
+ return BUG_TRAP_TYPE_NONE;
+}
+#else
+static inline enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
+{
+ return BUG_TRAP_TYPE_NONE;
+}
+#endif /* CONFIG_CFI_CLANG */
+
static noinstr bool handle_bug(struct pt_regs *regs)
{
bool handled = false;
@@ -312,7 +369,8 @@ static noinstr bool handle_bug(struct pt_regs *regs)
*/
if (regs->flags & X86_EFLAGS_IF)
raw_local_irq_enable();
- if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+ if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
+ handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
regs->ip += LEN_UD2;
handled = true;
}
--
2.36.0.550.gb090851708-goog