[RFC PATCH v2 10/21] arm64: Add CFI error handling

Sami Tolvanen
2022-05-13T20:22:26Z

With -fsanitize=kcfi, CFI always traps. Add arm64 support for handling
CFI failures. The registers containing the target address and the
expected type are encoded in the first ten bits of the ESR as follows:

- 0-4: n, where the register Xn contains the target address
- 5-9: m, where the register Wm contains the type hash

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
arch/arm64/include/asm/brk-imm.h | 6 +++++
arch/arm64/kernel/traps.c | 46 +++++++++++++++++++++++++++++---
2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/brk-imm.h b/arch/arm64/include/asm/brk-imm.h
index ec7720dbe2c8..6e000113e508 100644
--- a/arch/arm64/include/asm/brk-imm.h
+++ b/arch/arm64/include/asm/brk-imm.h
@@ -17,6 +17,7 @@
* 0x401: for compile time BRK instruction
* 0x800: kernel-mode BUG() and WARN() traps
* 0x9xx: tag-based KASAN trap (allowed values 0x900 - 0x9ff)
+ * 0x8xxx: Control-Flow Integrity traps
*/
#define KPROBES_BRK_IMM 0x004
#define UPROBES_BRK_IMM 0x005
@@ -28,4 +29,9 @@
#define KASAN_BRK_IMM 0x900
#define KASAN_BRK_MASK 0x0ff

+#define CFI_BRK_IMM_TARGET GENMASK(4, 0)
+#define CFI_BRK_IMM_TYPE GENMASK(9, 5)
+#define CFI_BRK_IMM_BASE 0x8000
+#define CFI_BRK_IMM_MASK (CFI_BRK_IMM_TARGET | CFI_BRK_IMM_TYPE)
+
#endif
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 0529fd57567e..17b083b683f4 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 +991,37 @@ static struct break_hook bug_break_hook = {
.imm = BUG_BRK_IMM,
};

+#ifdef CONFIG_CFI_CLANG
+static int cfi_handler(struct pt_regs *regs, unsigned int esr)
+{
+ unsigned long target, type;
+
+ target = pt_regs_read_reg(regs, FIELD_GET(CFI_BRK_IMM_TARGET, esr));
+ type = pt_regs_read_reg(regs, FIELD_GET(CFI_BRK_IMM_TYPE, esr));
+
+ switch (report_cfi_failure(regs, regs->pc, target, type)) {
+ 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_BASE,
+ .mask = CFI_BRK_IMM_MASK,
+};
+#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",
@@ -1051,6 +1083,9 @@ static struct break_hook kasan_break_hook = {
};
#endif

+
+#define esr_comment(esr) ((esr) & ESR_ELx_BRK64_ISS_COMMENT_MASK)
+
/*
* Initial handler for AArch64 BRK exceptions
* This handler only used until debug_traps_init().
@@ -1058,10 +1093,12 @@ static struct break_hook kasan_break_hook = {
int __init early_brk64(unsigned long addr, unsigned int esr,
struct pt_regs *regs)
{
+#ifdef CONFIG_CFI_CLANG
+ if ((esr_comment(esr) & ~CFI_BRK_IMM_MASK) == CFI_BRK_IMM_BASE)
+ return cfi_handler(regs, esr) != DBG_HOOK_HANDLED;
+#endif
#ifdef CONFIG_KASAN_SW_TAGS
- unsigned int comment = esr & ESR_ELx_BRK64_ISS_COMMENT_MASK;
-
- if ((comment & ~KASAN_BRK_MASK) == KASAN_BRK_IMM)
+ if ((esr_comment(esr) & ~KASAN_BRK_MASK) == KASAN_BRK_IMM)
return kasan_handler(regs, esr) != DBG_HOOK_HANDLED;
#endif
return bug_handler(regs, esr) != DBG_HOOK_HANDLED;
@@ -1070,6 +1107,9 @@ 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);
--
2.36.0.550.gb090851708-goog