arm64: Unmask Debug + SError in do_notify_resume()

When returning to a user context, the arm64 entry code masks all DAIF
exceptions before handling pending work in exit_to_user_mode_prepare()
and do_notify_resume(), where it will transiently unmask all DAIF
exceptions. This is a holdover from the old entry assembly, which
conservatively masked all DAIF exceptions, and it's only necessary to
mask interrupts at this point during the exception return path, so long
as we subsequently mask all DAIF exceptions before the actual exception
return.

While most DAIF manipulation follows a save...restore sequence, the
manipulation in do_notify_resume() is the other way around, unmasking
all DAIF exceptions before masking them again. This is unfortunate as we
unnecessarily mask Debug and SError exceptions, and it would be nice to
remove this special case to make DAIF manipulation simpler and most
consistent.

This patch changes exit_to_user_mode_prepare() and do_notify_resume() to
only mask interrupts while handling pending work, masking other DAIF
exceptions after this has completed. This removes the unusual DAIF
manipulation and allows Debug and SError exceptions to be taken for a
slightly longer window during the exception return path.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240206123848.1696480-4-mark.rutland@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Itaru Kitayama <itaru.kitayama@linux.dev>

arch/arm64/kernel/entry-common.c

@@ -130,7 +130,7 @@ static __always_inline void __exit_to_user_mode(void)
 static void do_notify_resume(struct pt_regs *regs, unsigned long thread_flags)
 {
 	do {
-		local_daif_restore(DAIF_PROCCTX);
+		local_irq_enable();
 
 		if (thread_flags & _TIF_NEED_RESCHED)
 			schedule();
@@ -153,7 +153,7 @@ static void do_notify_resume(struct pt_regs *regs, unsigned long thread_flags)
 		if (thread_flags & _TIF_FOREIGN_FPSTATE)
 			fpsimd_restore_current_state();
 
-		local_daif_mask();
+		local_irq_disable();
 		thread_flags = read_thread_flags();
 	} while (thread_flags & _TIF_WORK_MASK);
 }
@@ -162,12 +162,14 @@ static __always_inline void exit_to_user_mode_prepare(struct pt_regs *regs)
 {
 	unsigned long flags;
 
-	local_daif_mask();
+	local_irq_disable();
 
 	flags = read_thread_flags();
 	if (unlikely(flags & _TIF_WORK_MASK))
 		do_notify_resume(regs, flags);
 
+	local_daif_mask();
+
 	lockdep_sys_exit();
 }