[PATCH v2] arm64: debug: always unmask interrupts in el0_softstp()

[Ada Couprie Diaz]

EL0 exception handlers should always call `exit_to_user_mode()` with
interrupts unmasked.
When handling a completed single-step, we skip the if block and
`local_daif_restore(DAIF_PROCCTX)` never gets called,
which ends up calling `exit_to_user_mode()` with interrupts masked.

This is broken if pNMI is in use, as `do_notify_resume()` will try
to enable interrupts, but `local_irq_enable()` will only change the PMR,
leaving interrupts masked via DAIF.

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: 0ac7584c08ce ("arm64: debug: split single stepping exception entry")
Cc: <stable at vger.kernel.org> # 6.17
Signed-off-by: Ada Couprie Diaz <ada.coupriediaz at arm.com>
----
This was already broken in a similar fashion in kernels prior to v6.17,
as `local_daif_restore()` was called _after_ the debug handlers, with some
calling `send_user_sigtrap()` which would unmask interrupts via PMR
while leaving them masked by DAIF.

v2: Add missing semicolon so that the patch compiles.
My mistake for rushing a fix late, apologies.
---
 arch/arm64/kernel/entry-common.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 2b0c5925502e..8473fe4791bc 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -832,6 +832,7 @@ 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();
 
@@ -842,10 +843,11 @@ 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);
-	}
+
 	exit_to_user_mode(regs);
 }
 

base-commit: 449d48b1b99fdaa076166e200132705ac2bee711
-- 
2.43.0