arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs

Scheduling a 32-bit application on a 64-bit-only CPU is a bad idea.

Ensure that 32-bit applications always take the slow-path when returning
to userspace on a system with mismatched support at EL0, so that we can
avoid trying to run on a 64-bit-only CPU and force a SIGKILL instead.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210608180313.11502-5-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/process.c

@@ -527,6 +527,15 @@ static void erratum_1418040_thread_switch(struct task_struct *prev,
 	write_sysreg(val, cntkctl_el1);
 }
 
+static void compat_thread_switch(struct task_struct *next)
+{
+	if (!is_compat_thread(task_thread_info(next)))
+		return;
+
+	if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
+		set_tsk_thread_flag(next, TIF_NOTIFY_RESUME);
+}
+
 static void update_sctlr_el1(u64 sctlr)
 {
 	/*
@@ -568,6 +577,7 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
 	ssbs_thread_switch(next);
 	erratum_1418040_thread_switch(prev, next);
 	ptrauth_thread_switch_user(next);
+	compat_thread_switch(next);
 
 	/*
 	 * Complete any pending TLB or cache maintenance on this CPU in case
@@ -633,8 +643,15 @@ unsigned long arch_align_stack(unsigned long sp)
  */
 void arch_setup_new_exec(void)
 {
-	current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
+	unsigned long mmflags = 0;
 
+	if (is_compat_task()) {
+		mmflags = MMCF_AARCH32;
+		if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
+			set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+	}
+
+	current->mm->context.flags = mmflags;
 	ptrauth_thread_init_user();
 	mte_thread_init_user();
 

arch/arm64/kernel/signal.c

@@ -911,6 +911,19 @@ static void do_signal(struct pt_regs *regs)
 	restore_saved_sigmask();
 }
 
+static bool cpu_affinity_invalid(struct pt_regs *regs)
+{
+	if (!compat_user_mode(regs))
+		return false;
+
+	/*
+	 * We're preemptible, but a reschedule will cause us to check the
+	 * affinity again.
+	 */
+	return !cpumask_test_cpu(raw_smp_processor_id(),
+				 system_32bit_el0_cpumask());
+}
+
 asmlinkage void do_notify_resume(struct pt_regs *regs,
 				 unsigned long thread_flags)
 {
@@ -938,6 +951,19 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
 			if (thread_flags & _TIF_NOTIFY_RESUME) {
 				tracehook_notify_resume(regs);
 				rseq_handle_notify_resume(NULL, regs);
+
+				/*
+				 * If we reschedule after checking the affinity
+				 * then we must ensure that TIF_NOTIFY_RESUME
+				 * is set so that we check the affinity again.
+				 * Since tracehook_notify_resume() clears the
+				 * flag, ensure that the compiler doesn't move
+				 * it after the affinity check.
+				 */
+				barrier();
+
+				if (cpu_affinity_invalid(regs))
+					force_sig(SIGKILL);
 			}
 
 			if (thread_flags & _TIF_FOREIGN_FPSTATE)