arm64: debug: split hardware breakpoint exception entry

Currently all debug exceptions share common entry code and are routed
to `do_debug_exception()`, which calls dynamically-registered
handlers for each specific debug exception. This is unfortunate as
different debug exceptions have different entry handling requirements,
and it would be better to handle these distinct requirements earlier.

Hardware breakpoints exceptions are generated by the hardware after user
configuration. As such, they can be exploited when training branch
predictors outside of the userspace VA range: they still need to call
`arm64_apply_bp_hardening()` if needed to mitigate against this attack.

However, they do not need to handle the Cortex-A76 erratum #1463225 as
it only applies to single stepping exceptions.
It does not set an address in FAR_EL1 either, only the hardware
watchpoint does.

As the hardware breakpoint handler only returns 0 and never triggers
the call to `arm64_notify_die()`, we can call it directly from
`entry-common.c`.
Split the hardware breakpoint exception entry, adjust
the function signature, and handling of the Cortex-A76 erratum to fit
the behaviour of the exception.

Move the call to `arm64_apply_bp_hardening()` to `entry-common.c` so that
we can do it as early as possible, and only for the exceptions coming
from EL0, where it is needed.
This is safe to do as it is `noinstr`, as are all the functions it
may call. `el0_ia()` and `el0_pc()` already call it this way.

Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Tested-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Reviewed-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20250707114109.35672-8-ada.coupriediaz@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: Ada Couprie Diaz

arch/arm64/include/asm/exception.h

@@ -61,6 +61,11 @@ void do_el0_gcs(struct pt_regs *regs, unsigned long esr);
 void do_el1_gcs(struct pt_regs *regs, unsigned long esr);
 void do_debug_exception(unsigned long addr_if_watchpoint, unsigned long esr,
 			struct pt_regs *regs);
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+void do_breakpoint(unsigned long esr, struct pt_regs *regs);
+#else
+static inline void do_breakpoint(unsigned long esr, struct pt_regs *regs) {}
+#endif /* CONFIG_HAVE_HW_BREAKPOINT */
 void do_fpsimd_acc(unsigned long esr, struct pt_regs *regs);
 void do_sve_acc(unsigned long esr, struct pt_regs *regs);
 void do_sme_acc(unsigned long esr, struct pt_regs *regs);

arch/arm64/kernel/entry-common.c

@@ -526,6 +526,15 @@ static void noinstr el1_mops(struct pt_regs *regs, unsigned long esr)
 	exit_to_kernel_mode(regs);
 }
 
+static void noinstr el1_breakpt(struct pt_regs *regs, unsigned long esr)
+{
+	arm64_enter_el1_dbg(regs);
+	debug_exception_enter(regs);
+	do_breakpoint(esr, regs);
+	debug_exception_exit(regs);
+	arm64_exit_el1_dbg(regs);
+}
+
 static void noinstr el1_dbg(struct pt_regs *regs, unsigned long esr)
 {
 	unsigned long far = read_sysreg(far_el1);
@@ -575,6 +584,8 @@ asmlinkage void noinstr el1h_64_sync_handler(struct pt_regs *regs)
 		el1_mops(regs, esr);
 		break;
 	case ESR_ELx_EC_BREAKPT_CUR:
+		el1_breakpt(regs, esr);
+		break;
 	case ESR_ELx_EC_SOFTSTP_CUR:
 	case ESR_ELx_EC_WATCHPT_CUR:
 	case ESR_ELx_EC_BRK64:
@@ -769,6 +780,19 @@ static void noinstr el0_inv(struct pt_regs *regs, unsigned long esr)
 	exit_to_user_mode(regs);
 }
 
+static void noinstr el0_breakpt(struct pt_regs *regs, unsigned long esr)
+{
+	if (!is_ttbr0_addr(regs->pc))
+		arm64_apply_bp_hardening();
+
+	enter_from_user_mode(regs);
+	debug_exception_enter(regs);
+	do_breakpoint(esr, regs);
+	debug_exception_exit(regs);
+	local_daif_restore(DAIF_PROCCTX);
+	exit_to_user_mode(regs);
+}
+
 static void noinstr el0_dbg(struct pt_regs *regs, unsigned long esr)
 {
 	/* Only watchpoints write FAR_EL1, otherwise its UNKNOWN */
@@ -848,6 +872,8 @@ asmlinkage void noinstr el0t_64_sync_handler(struct pt_regs *regs)
 		el0_gcs(regs, esr);
 		break;
 	case ESR_ELx_EC_BREAKPT_LOW:
+		el0_breakpt(regs, esr);
+		break;
 	case ESR_ELx_EC_SOFTSTP_LOW:
 	case ESR_ELx_EC_WATCHPT_LOW:
 	case ESR_ELx_EC_BRK64:
@@ -968,6 +994,8 @@ asmlinkage void noinstr el0t_32_sync_handler(struct pt_regs *regs)
 		el0_cp15(regs, esr);
 		break;
 	case ESR_ELx_EC_BREAKPT_LOW:
+		el0_breakpt(regs, esr);
+		break;
 	case ESR_ELx_EC_SOFTSTP_LOW:
 	case ESR_ELx_EC_WATCHPT_LOW:
 	case ESR_ELx_EC_BKPT32:

arch/arm64/kernel/hw_breakpoint.c

@@ -22,6 +22,7 @@
 #include <asm/current.h>
 #include <asm/debug-monitors.h>
 #include <asm/esr.h>
+#include <asm/exception.h>
 #include <asm/hw_breakpoint.h>
 #include <asm/traps.h>
 #include <asm/cputype.h>
@@ -618,8 +619,7 @@ NOKPROBE_SYMBOL(toggle_bp_registers);
 /*
  * Debug exception handlers.
  */
-static int breakpoint_handler(unsigned long unused, unsigned long esr,
-			      struct pt_regs *regs)
+void do_breakpoint(unsigned long esr, struct pt_regs *regs)
 {
 	int i, step = 0, *kernel_step;
 	u32 ctrl_reg;
@@ -662,15 +662,15 @@ static int breakpoint_handler(unsigned long unused, unsigned long esr,
 	}
 
 	if (!step)
-		return 0;
+		return;
 
 	if (user_mode(regs)) {
 		debug_info->bps_disabled = 1;
 		toggle_bp_registers(AARCH64_DBG_REG_BCR, DBG_ACTIVE_EL0, 0);
 
 		/* If we're already stepping a watchpoint, just return. */
 		if (debug_info->wps_disabled)
-			return 0;
+			return;
 
 		if (test_thread_flag(TIF_SINGLESTEP))
 			debug_info->suspended_step = 1;
@@ -681,7 +681,7 @@ static int breakpoint_handler(unsigned long unused, unsigned long esr,
 		kernel_step = this_cpu_ptr(&stepping_kernel_bp);
 
 		if (*kernel_step != ARM_KERNEL_STEP_NONE)
-			return 0;
+			return;
 
 		if (kernel_active_single_step()) {
 			*kernel_step = ARM_KERNEL_STEP_SUSPEND;
@@ -690,10 +690,8 @@ static int breakpoint_handler(unsigned long unused, unsigned long esr,
 			kernel_enable_single_step(regs);
 		}
 	}
-
-	return 0;
 }
-NOKPROBE_SYMBOL(breakpoint_handler);
+NOKPROBE_SYMBOL(do_breakpoint);
 
 /*
  * Arm64 hardware does not always report a watchpoint hit address that matches
@@ -988,8 +986,6 @@ static int __init arch_hw_breakpoint_init(void)
 		core_num_brps, core_num_wrps);
 
 	/* Register debug fault handlers. */
-	hook_debug_fault_code(DBG_ESR_EVT_HWBP, breakpoint_handler, SIGTRAP,
-			      TRAP_HWBKPT, "hw-breakpoint handler");
 	hook_debug_fault_code(DBG_ESR_EVT_HWWP, watchpoint_handler, SIGTRAP,
 			      TRAP_HWBKPT, "hw-watchpoint handler");