arm64: debug: always unmask interrupts in el0_softstp()

We intend that EL0 exception handlers unmask all DAIF exceptions
before calling exit_to_user_mode().

When completing single-step of a suspended breakpoint, we do not call
local_daif_restore(DAIF_PROCCTX) before calling exit_to_user_mode(),
leaving all DAIF exceptions masked.

When pseudo-NMIs are not in use this is benign.

When pseudo-NMIs are in use, this is unsound. At this point interrupts
are masked by both DAIF.IF and PMR_EL1, and subsequent irq flag
manipulation may not work correctly. For example, a subsequent
local_irq_enable() within exit_to_user_mode_loop() will only unmask
interrupts via PMR_EL1 (leaving those masked via DAIF.IF), and
anything depending on interrupts being unmasked (e.g. delivery of
signals) will not work correctly.

This was detected by CONFIG_ARM64_DEBUG_PRIORITY_MASKING.

Move the call to `try_step_suspended_breakpoints()` outside of the check
so that interrupts can be unmasked even if we don't call the step handler.

Fixes: 0ac7584 ("arm64: debug: split single stepping exception entry")
Cc: <stable@vger.kernel.org> # 6.17
Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
[catalin.marinas@arm.com: added Mark's rewritten commit log and some whitespace]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: Ada Couprie Diaz

arch/arm64/kernel/entry-common.c

@@ -697,6 +697,8 @@ static void noinstr el0_breakpt(struct pt_regs *regs, unsigned long esr)
 
 static void noinstr el0_softstp(struct pt_regs *regs, unsigned long esr)
 {
+	bool step_done;
+
 	if (!is_ttbr0_addr(regs->pc))
 		arm64_apply_bp_hardening();
 
@@ -707,10 +709,10 @@ static void noinstr el0_softstp(struct pt_regs *regs, unsigned long esr)
 	 * If we are stepping a suspended breakpoint there's nothing more to do:
 	 * the single-step is complete.
 	 */
-	if (!try_step_suspended_breakpoints(regs)) {
-		local_daif_restore(DAIF_PROCCTX);
+	step_done = try_step_suspended_breakpoints(regs);
+	local_daif_restore(DAIF_PROCCTX);
+	if (!step_done)
 		do_el0_softstep(esr, regs);
-	}
 	arm64_exit_to_user_mode(regs);
 }