[PATCH v2 09/11] arm64: debug: split brk64 exception entry

[Ada Couprie Diaz]

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.

The BRK64 instruction can only be triggered by a BRK instruction. Thus,
we know that the PC is a legitimate address and isn't being used to train
a branch predictor with a bogus address : we don't need to call
`arm64_apply_bp_hardening()`.

We do not need to handle the Cortex-A76 erratum #1463225 either, as it
only relevant for single stepping at EL1.
BRK64 does not write FAR_EL1 either, as only hardware watchpoints do so.

Split the BRK64 exception entry, adjust the function signature, and its
behaviour to match the lack of needed mitigations.

Given that the break hook registration is handled statically in
`call_break_hook` since
(arm64: debug: call software break handlers statically)
and that we now bypass the exception handler registration, this change
renders `early_brk64` redundant : its functionality is now handled through
the post-init path.

This also removes the last usage of `el1_dbg()`.

Signed-off-by: Ada Couprie Diaz <ada.coupriediaz at arm.com>
---
 arch/arm64/include/asm/exception.h |  1 +
 arch/arm64/kernel/debug-monitors.c | 16 ++++++++++------
 arch/arm64/kernel/entry-common.c   | 21 ++++++++++++++++-----
 3 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h
index eb465c375d34..af9b521e35f9 100644
--- a/arch/arm64/include/asm/exception.h
+++ b/arch/arm64/include/asm/exception.h
@@ -65,6 +65,7 @@ void do_breakpoint(unsigned long esr, struct pt_regs *regs);
 void do_softstep(unsigned long esr, struct pt_regs *regs);
 void do_watchpoint(unsigned long addr, unsigned long esr,
 			struct pt_regs *regs);
+void do_brk64(unsigned long esr, struct pt_regs *regs);
 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);
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index d3cb8f5da51b..94779c957b70 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -274,8 +274,7 @@ static int call_break_hook(struct pt_regs *regs, unsigned long esr)
 }
 NOKPROBE_SYMBOL(call_break_hook);
 
-static int brk_handler(unsigned long unused, unsigned long esr,
-		       struct pt_regs *regs)
+static int brk_handler(unsigned long esr, struct pt_regs *regs)
 {
 	if (call_break_hook(regs, esr) == DBG_HOOK_HANDLED)
 		return 0;
@@ -291,6 +290,14 @@ static int brk_handler(unsigned long unused, unsigned long esr,
 }
 NOKPROBE_SYMBOL(brk_handler);
 
+void do_brk64(unsigned long esr, struct pt_regs *regs)
+{
+	if (brk_handler(esr, regs))
+		arm64_notify_die("BRK handler", regs, SIGTRAP, TRAP_BRKPT, regs->pc,
+			esr);
+}
+NOKPROBE_SYMBOL(do_brk64);
+
 int aarch32_break_handler(struct pt_regs *regs)
 {
 	u32 arm_instr;
@@ -331,10 +338,7 @@ int aarch32_break_handler(struct pt_regs *regs)
 NOKPROBE_SYMBOL(aarch32_break_handler);
 
 void __init debug_traps_init(void)
-{
-	hook_debug_fault_code(DBG_ESR_EVT_BRK, brk_handler, SIGTRAP,
-			      TRAP_BRKPT, "BRK handler");
-}
+{}
 
 /* Re-enable single step for syscall restarting. */
 void user_rewind_single_step(struct task_struct *task)
diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 6e70130d2741..424d072659b5 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -542,11 +542,12 @@ static void noinstr el1_watchpt(struct pt_regs *regs, unsigned long esr)
 	arm64_exit_el1_dbg(regs);
 }
 
-static void noinstr el1_dbg(struct pt_regs *regs, unsigned long esr)
+static void noinstr el1_brk64(struct pt_regs *regs, unsigned long esr)
 {
 	arm64_enter_el1_dbg(regs);
-	if (!cortex_a76_erratum_1463225_debug_handler(regs))
-		do_debug_exception(far, esr, regs);
+	debug_exception_enter(regs);
+	do_brk64(esr, regs);
+	debug_exception_exit(regs);
 	arm64_exit_el1_dbg(regs);
 }
 
@@ -598,7 +599,7 @@ asmlinkage void noinstr el1h_64_sync_handler(struct pt_regs *regs)
 		el1_watchpt(regs, esr);
 		break;
 	case ESR_ELx_EC_BRK64:
-		el1_dbg(regs, esr);
+		el1_brk64(regs, esr);
 		break;
 	case ESR_ELx_EC_FPAC:
 		el1_fpac(regs, esr);
@@ -834,6 +835,16 @@ static void noinstr el0_watchpt(struct pt_regs *regs, unsigned long esr)
 	exit_to_user_mode(regs);
 }
 
+static void noinstr el0_brk64(struct pt_regs *regs, unsigned long esr)
+{
+	enter_from_user_mode(regs);
+	debug_exception_enter(regs);
+	do_brk64(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 */
@@ -921,7 +932,7 @@ asmlinkage void noinstr el0t_64_sync_handler(struct pt_regs *regs)
 		el0_watchpt(regs, esr);
 		break;
 	case ESR_ELx_EC_BRK64:
-		el0_dbg(regs, esr);
+		el0_brk64(regs, esr);
 		break;
 	case ESR_ELx_EC_FPAC:
 		el0_fpac(regs, esr);
-- 
2.43.0