[RFC PATCH 09/21] arm64: Add CFI error handling

Sami Tolvanen
2022-04-29T20:44:00Z

With -fsanitize=kcfi, CFI always traps. Add arm64 support for handling
CFI failures and determining the target address.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
arch/arm64/include/asm/brk-imm.h | 2 ++
arch/arm64/include/asm/insn.h | 1 +
arch/arm64/kernel/traps.c | 57 ++++++++++++++++++++++++++++++++
3 files changed, 60 insertions(+)

diff --git a/arch/arm64/include/asm/brk-imm.h b/arch/arm64/include/asm/brk-imm.h
index ec7720dbe2c8..3a50b70b4404 100644
--- a/arch/arm64/include/asm/brk-imm.h
+++ b/arch/arm64/include/asm/brk-imm.h
@@ -16,6 +16,7 @@
* 0x400: for dynamic BRK instruction
* 0x401: for compile time BRK instruction
* 0x800: kernel-mode BUG() and WARN() traps
+ * 0x801: Control-Flow Integrity traps
* 0x9xx: tag-based KASAN trap (allowed values 0x900 - 0x9ff)
*/
#define KPROBES_BRK_IMM 0x004
@@ -25,6 +26,7 @@
#define KGDB_DYN_DBG_BRK_IMM 0x400
#define KGDB_COMPILED_DBG_BRK_IMM 0x401
#define BUG_BRK_IMM 0x800
+#define CFI_BRK_IMM 0x801
#define KASAN_BRK_IMM 0x900
#define KASAN_BRK_MASK 0x0ff

diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
index 1e5760d567ae..12225bdfa776 100644
--- a/arch/arm64/include/asm/insn.h
+++ b/arch/arm64/include/asm/insn.h
@@ -334,6 +334,7 @@ __AARCH64_INSN_FUNCS(store_pre, 0x3FE00C00, 0x38000C00)
__AARCH64_INSN_FUNCS(load_pre, 0x3FE00C00, 0x38400C00)
__AARCH64_INSN_FUNCS(store_post, 0x3FE00C00, 0x38000400)
__AARCH64_INSN_FUNCS(load_post, 0x3FE00C00, 0x38400400)
+__AARCH64_INSN_FUNCS(ldur, 0x3FE00C00, 0x38400000)
__AARCH64_INSN_FUNCS(str_reg, 0x3FE0EC00, 0x38206800)
__AARCH64_INSN_FUNCS(ldadd, 0x3F20FC00, 0x38200000)
__AARCH64_INSN_FUNCS(ldclr, 0x3F20FC00, 0x38201000)
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 0529fd57567e..b524411ba663 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -26,6 +26,7 @@
#include <linux/syscalls.h>
#include <linux/mm_types.h>
#include <linux/kasan.h>
+#include <linux/cfi.h>

#include <asm/atomic.h>
#include <asm/bug.h>
@@ -990,6 +991,55 @@ static struct break_hook bug_break_hook = {
.imm = BUG_BRK_IMM,
};

+#ifdef CONFIG_CFI_CLANG
+void *arch_get_cfi_target(unsigned long addr, struct pt_regs *regs)
+{
+ /* The expected CFI check instruction sequence:
+ * ldur    wA, [xN, #-4]
+ * movk    wB, #nnnnn
+ * movk    wB, #nnnnn, lsl #16
+ * cmp     wA, wB
+ * b.eq    .Ltmp1
+ * brk     #0x801 ; <- addr
+ * .Ltmp1:
+ *
+ * Therefore, the target address is in the xN register, which we can
+ * decode from the ldur instruction.
+ */
+ u32 insn, rn;
+ void *p = (void *)(addr - 5 * AARCH64_INSN_SIZE);
+
+ if (aarch64_insn_read(p, &insn) || !aarch64_insn_is_ldur(insn))
+ return NULL;
+
+ rn = aarch64_insn_decode_register(AARCH64_INSN_REGTYPE_RN, insn);
+ return (void *)regs->regs[rn];
+}
+
+static int cfi_handler(struct pt_regs *regs, unsigned int esr)
+{
+ switch (report_cfi(regs->pc, regs)) {
+ case BUG_TRAP_TYPE_BUG:
+ die("Oops - CFI", regs, 0);
+ break;
+
+ case BUG_TRAP_TYPE_WARN:
+ break;
+
+ default:
+ return DBG_HOOK_ERROR;
+ }
+
+ arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
+ return DBG_HOOK_HANDLED;
+}
+
+static struct break_hook cfi_break_hook = {
+ .fn = cfi_handler,
+ .imm = CFI_BRK_IMM,
+};
+#endif /* CONFIG_CFI_CLANG */
+
static int reserved_fault_handler(struct pt_regs *regs, unsigned int esr)
{
pr_err("%s generated an invalid instruction at %pS!\n",
@@ -1063,6 +1113,10 @@ int __init early_brk64(unsigned long addr, unsigned int esr,

if ((comment & ~KASAN_BRK_MASK) == KASAN_BRK_IMM)
return kasan_handler(regs, esr) != DBG_HOOK_HANDLED;
+#endif
+#ifdef CONFIG_CFI_CLANG
+ if ((esr & ESR_ELx_BRK64_ISS_COMMENT_MASK) == CFI_BRK_IMM)
+ return cfi_handler(regs, esr) != DBG_HOOK_HANDLED;
#endif
return bug_handler(regs, esr) != DBG_HOOK_HANDLED;
}
@@ -1070,6 +1124,9 @@ int __init early_brk64(unsigned long addr, unsigned int esr,
void __init trap_init(void)
{
register_kernel_break_hook(&bug_break_hook);
+#ifdef CONFIG_CFI_CLANG
+ register_kernel_break_hook(&cfi_break_hook);
+#endif
register_kernel_break_hook(&fault_break_hook);
#ifdef CONFIG_KASAN_SW_TAGS
register_kernel_break_hook(&kasan_break_hook);
--
2.36.0.464.gb9c8b46e94-goog