ARM: 9391/2: hw_breakpoint: Handle CFI breakpoints

This registers a breakpoint handler for the new breakpoint type
(0x03) inserted by LLVM CLANG for CFI breakpoints.

If we are in permissive mode, just print a backtrace and continue.

Example with CONFIG_CFI_PERMISSIVE enabled:

> echo CFI_FORWARD_PROTO > /sys/kernel/debug/provoke-crash/DIRECT
lkdtm: Performing direct entry CFI_FORWARD_PROTO
lkdtm: Calling matched prototype ...
lkdtm: Calling mismatched prototype ...
CFI failure at lkdtm_indirect_call+0x40/0x4c (target: 0x0; expected type: 0x00000000)
WARNING: CPU: 1 PID: 112 at lkdtm_indirect_call+0x40/0x4c
CPU: 1 PID: 112 Comm: sh Not tainted 6.8.0-rc1+ #150
Hardware name: ARM-Versatile Express
(...)
lkdtm: FAIL: survived mismatched prototype function call!
lkdtm: Unexpected! This kernel (6.8.0-rc1+ armv7l) was built with CONFIG_CFI_CLANG=y

As you can see the LKDTM test fails, but I expect that this would be
expected behaviour in the permissive mode.

We are currently not implementing target and type for the CFI
breakpoint as this requires additional operand bundling compiler
extensions.

CPUs without breakpoint support cannot handle breakpoints naturally,
in these cases the permissive mode will not work, CFI will fall over
on an undefined instruction:

Internal error: Oops - undefined instruction: 0 [#1] PREEMPT ARM
CPU: 0 PID: 186 Comm: ash Tainted: G W 6.9.0-rc1+ #7
Hardware name: Gemini (Device Tree)
PC is at lkdtm_indirect_call+0x38/0x4c
LR is at lkdtm_CFI_FORWARD_PROTO+0x30/0x6c

This is reasonable I think: it's the best CFI can do to ascertain
the the control flow is not broken on these CPUs.

Reviewed-by: Kees Cook <[e-mail]>
Tested-by: Kees Cook <[e-mail]>
Reviewed-by: Sami Tolvanen <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Signed-off-by: Russell King (Oracle) <[e-mail]>

arch/arm/include/asm/hw_breakpoint.h

···
 #define ARM_DSCR_MOE(x)			((x >> 2) & 0xf)
 #define ARM_ENTRY_BREAKPOINT		0x1
 #define ARM_ENTRY_ASYNC_WATCHPOINT	0x2
+#define ARM_ENTRY_CFI_BREAKPOINT	0x3
 #define ARM_ENTRY_SYNC_WATCHPOINT	0xa
 
 /* DSCR monitor/halting bits. */

arch/arm/kernel/hw_breakpoint.c

···
 #include <linux/perf_event.h>
 #include <linux/hw_breakpoint.h>
 #include <linux/smp.h>
+#include <linux/cfi.h>
 #include <linux/cpu_pm.h>
 #include <linux/coresight.h>
 
···
 	watchpoint_single_step_handler(addr);
 }
 
+#ifdef CONFIG_CFI_CLANG
+static void hw_breakpoint_cfi_handler(struct pt_regs *regs)
+{
+	/*
+	 * TODO: implementing target and type to pass to CFI using the more
+	 * elaborate report_cfi_failure() requires compiler work. To be able
+	 * to properly extract target information the compiler needs to
+	 * emit a stable instructions sequence for the CFI checks so we can
+	 * decode the instructions preceding the trap and figure out which
+	 * registers were used.
+	 */
+
+	switch (report_cfi_failure_noaddr(regs, instruction_pointer(regs))) {
+	case BUG_TRAP_TYPE_BUG:
+		die("Oops - CFI", regs, 0);
+		break;
+	case BUG_TRAP_TYPE_WARN:
+		/* Skip the breaking instruction */
+		instruction_pointer(regs) += 4;
+		break;
+	default:
+		die("Unknown CFI error", regs, 0);
+		break;
+	}
+}
+#else
+static void hw_breakpoint_cfi_handler(struct pt_regs *regs)
+{
+}
+#endif
+
 /*
  * Called from either the Data Abort Handler [watchpoint] or the
  * Prefetch Abort Handler [breakpoint] with interrupts disabled.
···
 		fallthrough;
 	case ARM_ENTRY_SYNC_WATCHPOINT:
 		watchpoint_handler(addr, fsr, regs);
+		break;
+	case ARM_ENTRY_CFI_BREAKPOINT:
+		hw_breakpoint_cfi_handler(regs);
 		break;
 	default:
 		ret = 1; /* Unhandled fault. */